diff --git a/lib/message_builder.rb b/lib/message_builder.rb
index df2ec95b..5130b864 100644
--- a/lib/message_builder.rb
+++ b/lib/message_builder.rb
@@ -26,9 +26,22 @@ def build
     end
   rescue StandardError => e
     puts "Error building message: #{e.message}"
+    alert_slack(e.message)
     nil
   end
 
+  def alert_slack(message)
+    slack_options =
+      {
+        icon_emoji: ":sad-seal:",
+        username: "Seal error",
+        channel: "#govuk-platform-security-reliability-team",
+      }
+
+    poster = Slack::Poster.new(ENV["SLACK_WEBHOOK"].to_s, slack_options)
+    poster.send_message("The Seal has encountered an error running in mode #{@mode}: #{message}")
+  end
+
   def rotten?(pull_request)
     age_in_weekdays = (pr_date(pull_request)...Date.today)
       .reject { |d| d.saturday? || d.sunday? }