diff --git a/bridges/relays/ethereum-client/src/client.rs b/bridges/relays/ethereum-client/src/client.rs
index b5fe3ab6bf179..30a62a400e1dd 100644
--- a/bridges/relays/ethereum-client/src/client.rs
+++ b/bridges/relays/ethereum-client/src/client.rs
@@ -16,8 +16,8 @@
 
 use crate::rpc::Ethereum;
 use crate::types::{
-	Address, Bytes, CallRequest, Header, HeaderWithTransactions, Receipt, SignedRawTx, Transaction, TransactionHash,
-	H256, U256,
+	Address, Bytes, CallRequest, Header, HeaderWithTransactions, Receipt, SignedRawTx, SyncState, Transaction,
+	TransactionHash, H256, U256,
 };
 use crate::{ConnectionParams, Error, Result};
 
@@ -25,6 +25,9 @@ use jsonrpsee::raw::RawClient;
 use jsonrpsee::transport::http::HttpTransportClient;
 use jsonrpsee::Client as RpcClient;
 
+/// Number of headers missing from the Ethereum node for us to consider node not synced.
+const MAJOR_SYNC_BLOCKS: u64 = 5;
+
 /// The client used to interact with an Ethereum node through RPC.
 #[derive(Clone)]
 pub struct Client {
@@ -53,6 +56,23 @@ impl Client {
 	pub fn reconnect(&mut self) {
 		self.client = Self::build_client(&self.params);
 	}
+}
+
+impl Client {
+	/// Returns true if client is connected to at least one peer and is in synced state.
+	pub async fn ensure_synced(&self) -> Result<()> {
+		match Ethereum::syncing(&self.client).await? {
+			SyncState::NotSyncing => Ok(()),
+			SyncState::Syncing(syncing) => {
+				let missing_headers = syncing.highest_block.saturating_sub(syncing.current_block);
+				if missing_headers > MAJOR_SYNC_BLOCKS.into() {
+					return Err(Error::ClientNotSynced(missing_headers));
+				}
+
+				Ok(())
+			}
+		}
+	}
 
 	/// Estimate gas usage for the given call.
 	pub async fn estimate_gas(&self, call_request: CallRequest) -> Result<U256> {
diff --git a/bridges/relays/ethereum-client/src/error.rs b/bridges/relays/ethereum-client/src/error.rs
index b02e5fecf5842..afcc3c678254e 100644
--- a/bridges/relays/ethereum-client/src/error.rs
+++ b/bridges/relays/ethereum-client/src/error.rs
@@ -16,6 +16,8 @@
 
 //! Ethereum node RPC errors.
 
+use crate::types::U256;
+
 use jsonrpsee::client::RequestError;
 use relay_utils::MaybeConnectionError;
 
@@ -40,6 +42,9 @@ pub enum Error {
 	InvalidSubstrateBlockNumber,
 	/// An invalid index has been received from an Ethereum node.
 	InvalidIncompleteIndex,
+	/// The client we're connected to is not synced, so we can't rely on its state. Contains
+	/// number of unsynced headers.
+	ClientNotSynced(U256),
 }
 
 impl From<RequestError> for Error {
@@ -50,7 +55,11 @@ impl From<RequestError> for Error {
 
 impl MaybeConnectionError for Error {
 	fn is_connection_error(&self) -> bool {
-		matches!(*self, Error::Request(RequestError::TransportError(_)))
+		matches!(
+			*self,
+			Error::Request(RequestError::TransportError(_))
+				| Error::ClientNotSynced(_),
+		)
 	}
 }
 
@@ -66,6 +75,9 @@ impl ToString for Error {
 			Self::IncompleteTransaction => "Incomplete Ethereum Transaction (missing required field - raw)".to_string(),
 			Self::InvalidSubstrateBlockNumber => "Received an invalid Substrate block from Ethereum Node".to_string(),
 			Self::InvalidIncompleteIndex => "Received an invalid incomplete index from Ethereum Node".to_string(),
+			Self::ClientNotSynced(missing_headers) => {
+				format!("Ethereum client is not synced: syncing {} headers", missing_headers)
+			}
 		}
 	}
 }
diff --git a/bridges/relays/ethereum-client/src/rpc.rs b/bridges/relays/ethereum-client/src/rpc.rs
index 9739d3edbe280..3fa4f6ceb9cd9 100644
--- a/bridges/relays/ethereum-client/src/rpc.rs
+++ b/bridges/relays/ethereum-client/src/rpc.rs
@@ -22,11 +22,14 @@
 #![allow(unused_variables)]
 
 use crate::types::{
-	Address, Bytes, CallRequest, Header, HeaderWithTransactions, Receipt, Transaction, TransactionHash, H256, U256, U64,
+	Address, Bytes, CallRequest, Header, HeaderWithTransactions, Receipt, SyncState, Transaction, TransactionHash,
+	H256, U256, U64,
 };
 
 jsonrpsee::rpc_api! {
 	pub(crate) Ethereum {
+		#[rpc(method = "eth_syncing", positional_params)]
+		fn syncing() -> SyncState;
 		#[rpc(method = "eth_estimateGas", positional_params)]
 		fn estimate_gas(call_request: CallRequest) -> U256;
 		#[rpc(method = "eth_blockNumber", positional_params)]
diff --git a/bridges/relays/ethereum-client/src/types.rs b/bridges/relays/ethereum-client/src/types.rs
index f64362ade0e88..1bb9233b82ea4 100644
--- a/bridges/relays/ethereum-client/src/types.rs
+++ b/bridges/relays/ethereum-client/src/types.rs
@@ -18,7 +18,7 @@
 
 use headers_relay::sync_types::SourceHeader;
 
-pub use web3::types::{Address, Bytes, CallRequest, H256, U128, U256, U64};
+pub use web3::types::{Address, Bytes, CallRequest, SyncState, H256, U128, U256, U64};
 
 /// When header is just received from the Ethereum node, we check that it has
 /// both number and hash fields filled.
diff --git a/bridges/relays/ethereum/src/ethereum_exchange.rs b/bridges/relays/ethereum/src/ethereum_exchange.rs
index 19d9f44cef0e9..92ba211535129 100644
--- a/bridges/relays/ethereum/src/ethereum_exchange.rs
+++ b/bridges/relays/ethereum/src/ethereum_exchange.rs
@@ -248,6 +248,10 @@ impl TargetClient<EthereumToSubstrateExchange> for SubstrateTransactionsTarget {
 	}
 
 	async fn best_finalized_header_id(&self) -> Result<EthereumHeaderId, RpcError> {
+		// we can't continue to relay exchange proofs if Substrate node is out of sync, because
+		// it may have already received (some of) proofs that we're going to relay
+		self.client.ensure_synced().await?;
+
 		self.client.best_ethereum_finalized_block().await
 	}
 
diff --git a/bridges/relays/ethereum/src/ethereum_sync_loop.rs b/bridges/relays/ethereum/src/ethereum_sync_loop.rs
index b84626d230c6d..c8741c2fe18a6 100644
--- a/bridges/relays/ethereum/src/ethereum_sync_loop.rs
+++ b/bridges/relays/ethereum/src/ethereum_sync_loop.rs
@@ -130,6 +130,9 @@ impl RelayClient for EthereumHeadersSource {
 #[async_trait]
 impl SourceClient<EthereumHeadersSyncPipeline> for EthereumHeadersSource {
 	async fn best_block_number(&self) -> Result<u64, RpcError> {
+		// we **CAN** continue to relay headers if Ethereum node is out of sync, because
+		// Substrate node may be missing headers that are already available at the Ethereum
+
 		self.client.best_block_number().await.map_err(Into::into)
 	}
 
@@ -204,6 +207,10 @@ impl RelayClient for SubstrateHeadersTarget {
 #[async_trait]
 impl TargetClient<EthereumHeadersSyncPipeline> for SubstrateHeadersTarget {
 	async fn best_header_id(&self) -> Result<EthereumHeaderId, RpcError> {
+		// we can't continue to relay headers if Substrate node is out of sync, because
+		// it may have already received (some of) headers that we're going to relay
+		self.client.ensure_synced().await?;
+
 		self.client.best_ethereum_block().await
 	}
 
diff --git a/bridges/relays/ethereum/src/substrate_sync_loop.rs b/bridges/relays/ethereum/src/substrate_sync_loop.rs
index 298fe0e592545..9e15b9223826a 100644
--- a/bridges/relays/ethereum/src/substrate_sync_loop.rs
+++ b/bridges/relays/ethereum/src/substrate_sync_loop.rs
@@ -131,6 +131,10 @@ impl RelayClient for EthereumHeadersTarget {
 #[async_trait]
 impl TargetClient<SubstrateHeadersSyncPipeline> for EthereumHeadersTarget {
 	async fn best_header_id(&self) -> Result<RialtoHeaderId, RpcError> {
+		// we can't continue to relay headers if Ethereum node is out of sync, because
+		// it may have already received (some of) headers that we're going to relay
+		self.client.ensure_synced().await?;
+
 		self.client.best_substrate_block(self.contract).await
 	}
 
diff --git a/bridges/relays/substrate-client/Cargo.toml b/bridges/relays/substrate-client/Cargo.toml
index 0e4b43bf7f6b6..320abf280f040 100644
--- a/bridges/relays/substrate-client/Cargo.toml
+++ b/bridges/relays/substrate-client/Cargo.toml
@@ -26,6 +26,7 @@ relay-utils = { path = "../utils" }
 frame-support = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
 frame-system = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
 pallet-balances = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
+sc-rpc-api = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
 sp-core = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
 sp-runtime = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
 sp-std = { git = "https://github.com/paritytech/substrate.git", branch = "master" }
diff --git a/bridges/relays/substrate-client/src/client.rs b/bridges/relays/substrate-client/src/client.rs
index cfa644bc8c71e..a5ddf143f1b03 100644
--- a/bridges/relays/substrate-client/src/client.rs
+++ b/bridges/relays/substrate-client/src/client.rs
@@ -104,6 +104,17 @@ impl<C: Chain> Client<C> {
 }
 
 impl<C: Chain> Client<C> {
+	/// Returns true if client is connected to at least one peer and is in synced state.
+	pub async fn ensure_synced(&self) -> Result<()> {
+		let health = Substrate::<C, _, _>::system_health(&self.client).await?;
+		let is_synced = !health.is_syncing && (!health.should_have_peers || health.peers > 0);
+		if is_synced {
+			Ok(())
+		} else {
+			Err(Error::ClientNotSynced(health))
+		}
+	}
+
 	/// Return hash of the genesis block.
 	pub fn genesis_hash(&self) -> &C::Hash {
 		&self.genesis_hash
diff --git a/bridges/relays/substrate-client/src/error.rs b/bridges/relays/substrate-client/src/error.rs
index c2930c21d217a..f0212ed501ae2 100644
--- a/bridges/relays/substrate-client/src/error.rs
+++ b/bridges/relays/substrate-client/src/error.rs
@@ -19,6 +19,7 @@
 use jsonrpsee::client::RequestError;
 use jsonrpsee::transport::ws::WsNewDnsError;
 use relay_utils::MaybeConnectionError;
+use sc_rpc_api::system::Health;
 
 /// Result type used by Substrate client.
 pub type Result<T> = std::result::Result<T, Error>;
@@ -38,6 +39,8 @@ pub enum Error {
 	UninitializedBridgePallet,
 	/// Account does not exist on the chain.
 	AccountDoesNotExist,
+	/// The client we're connected to is not synced, so we can't rely on its state.
+	ClientNotSynced(Health),
 	/// Custom logic error.
 	Custom(String),
 }
@@ -56,7 +59,11 @@ impl From<RequestError> for Error {
 
 impl MaybeConnectionError for Error {
 	fn is_connection_error(&self) -> bool {
-		matches!(*self, Error::Request(RequestError::TransportError(_)))
+		matches!(
+			*self,
+			Error::Request(RequestError::TransportError(_))
+				| Error::ClientNotSynced(_)
+		)
 	}
 }
 
@@ -74,6 +81,7 @@ impl ToString for Error {
 			Self::ResponseParseFailed(e) => e.what().to_string(),
 			Self::UninitializedBridgePallet => "The Substrate bridge pallet has not been initialized yet.".into(),
 			Self::AccountDoesNotExist => "Account does not exist on the chain".into(),
+			Self::ClientNotSynced(health) => format!("Substrate client is not synced: {}", health),
 			Self::Custom(e) => e.clone(),
 		}
 	}
diff --git a/bridges/relays/substrate-client/src/headers_source.rs b/bridges/relays/substrate-client/src/headers_source.rs
index 040cc08e99766..b347a1c9f57f2 100644
--- a/bridges/relays/substrate-client/src/headers_source.rs
+++ b/bridges/relays/substrate-client/src/headers_source.rs
@@ -73,6 +73,8 @@ where
 	P::Header: SourceHeader<C::Hash, C::BlockNumber>,
 {
 	async fn best_block_number(&self) -> Result<P::Number, Error> {
+		// we **CAN** continue to relay headers if source node is out of sync, because
+		// target node may be missing headers that are already available at the source
 		Ok(*self.client.best_header().await?.number())
 	}
 
diff --git a/bridges/relays/substrate-client/src/rpc.rs b/bridges/relays/substrate-client/src/rpc.rs
index 9c76f593e99b8..2e832b4018174 100644
--- a/bridges/relays/substrate-client/src/rpc.rs
+++ b/bridges/relays/substrate-client/src/rpc.rs
@@ -25,6 +25,7 @@ use crate::chain::Chain;
 
 use bp_message_lane::{LaneId, MessageNonce};
 use bp_runtime::InstanceId;
+use sc_rpc_api::system::Health;
 use sp_core::{
 	storage::{StorageData, StorageKey},
 	Bytes,
@@ -33,6 +34,8 @@ use sp_version::RuntimeVersion;
 
 jsonrpsee::rpc_api! {
 	pub(crate) Substrate<C: Chain> {
+		#[rpc(method = "system_health", positional_params)]
+		fn system_health() -> Health;
 		#[rpc(method = "chain_getHeader", positional_params)]
 		fn chain_get_header(block_hash: Option<C::Hash>) -> C::Header;
 		#[rpc(method = "chain_getFinalizedHead", positional_params)]
diff --git a/bridges/relays/substrate/src/headers_target.rs b/bridges/relays/substrate/src/headers_target.rs
index c962511270805..2b5f63a7feae3 100644
--- a/bridges/relays/substrate/src/headers_target.rs
+++ b/bridges/relays/substrate/src/headers_target.rs
@@ -73,6 +73,10 @@ where
 	P: SubstrateHeadersSyncPipeline<Completion = Justification, Extra = ()>,
 {
 	async fn best_header_id(&self) -> Result<HeaderIdOf<P>, SubstrateError> {
+		// we can't continue to relay headers if target node is out of sync, because
+		// it may have already received (some of) headers that we're going to relay
+		self.client.ensure_synced().await?;
+
 		let call = P::BEST_BLOCK_METHOD.into();
 		let data = Bytes(Vec::new());
 
diff --git a/bridges/relays/substrate/src/messages_source.rs b/bridges/relays/substrate/src/messages_source.rs
index b9a8d01a7c258..375c469fde1a9 100644
--- a/bridges/relays/substrate/src/messages_source.rs
+++ b/bridges/relays/substrate/src/messages_source.rs
@@ -99,6 +99,10 @@ where
 	P::TargetHeaderHash: Decode,
 {
 	async fn state(&self) -> Result<SourceClientState<P>, SubstrateError> {
+		// we can't continue to deliver confirmations if source node is out of sync, because
+		// it may have already received confirmations that we're going to deliver
+		self.client.ensure_synced().await?;
+
 		read_client_state::<_, P::TargetHeaderHash, P::TargetHeaderNumber>(
 			&self.client,
 			P::BEST_FINALIZED_TARGET_HEADER_ID_AT_SOURCE,
diff --git a/bridges/relays/substrate/src/messages_target.rs b/bridges/relays/substrate/src/messages_target.rs
index 0bc81d9be05a3..e8610aac9533c 100644
--- a/bridges/relays/substrate/src/messages_target.rs
+++ b/bridges/relays/substrate/src/messages_target.rs
@@ -95,6 +95,10 @@ where
 	P::SourceHeaderHash: Decode,
 {
 	async fn state(&self) -> Result<TargetClientState<P>, SubstrateError> {
+		// we can't continue to deliver messages if target node is out of sync, because
+		// it may have already received (some of) messages that we're going to deliver
+		self.client.ensure_synced().await?;
+
 		read_client_state::<_, P::SourceHeaderHash, P::SourceHeaderNumber>(
 			&self.client,
 			P::BEST_FINALIZED_SOURCE_HEADER_ID_AT_TARGET,