diff --git a/bridges/relays/bin-substrate/src/cli/mod.rs b/bridges/relays/bin-substrate/src/cli/mod.rs
index 6418848c29..5419f82637 100644
--- a/bridges/relays/bin-substrate/src/cli/mod.rs
+++ b/bridges/relays/bin-substrate/src/cli/mod.rs
@@ -27,6 +27,7 @@ use structopt::{clap::arg_enum, StructOpt};
 
 mod init_bridge;
 mod relay_headers;
+mod relay_messages;
 
 /// Parse relay CLI args.
 pub fn parse_args() -> Command {
@@ -46,7 +47,7 @@ pub enum Command {
 	///
 	/// Ties up to `Messages` pallets on both chains and starts relaying messages.
 	/// Requires the header relay to be already running.
-	RelayMessages(RelayMessages),
+	RelayMessages(relay_messages::RelayMessages),
 	/// Initialize on-chain bridge pallet with current header data.
 	///
 	/// Sends initialization transaction to bootstrap the bridge with current finalized block data.
@@ -90,23 +91,6 @@ impl Command {
 	}
 }
 
-/// Start message relayer process.
-#[derive(StructOpt)]
-pub enum RelayMessages {
-	#[structopt(flatten)]
-	RialtoMillau(rialto_millau::RelayMessages),
-}
-
-impl RelayMessages {
-	/// Run the command.
-	pub async fn run(self) -> anyhow::Result<()> {
-		match self {
-			Self::RialtoMillau(arg) => arg.run().await?,
-		}
-		Ok(())
-	}
-}
-
 /// Send bridge message.
 #[derive(StructOpt)]
 pub enum SendMessage {
diff --git a/bridges/relays/bin-substrate/src/cli/relay_messages.rs b/bridges/relays/bin-substrate/src/cli/relay_messages.rs
new file mode 100644
index 0000000000..80cb0654a3
--- /dev/null
+++ b/bridges/relays/bin-substrate/src/cli/relay_messages.rs
@@ -0,0 +1,97 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::cli::{
+	CliChain, HexLaneId, PrometheusParams, SourceConnectionParams, SourceSigningParams, TargetConnectionParams,
+	TargetSigningParams,
+};
+use structopt::{clap::arg_enum, StructOpt};
+
+/// Start messages relayer process.
+#[derive(StructOpt)]
+pub struct RelayMessages {
+	/// A bridge instance to relay messages for.
+	#[structopt(possible_values = &RelayMessagesBridge::variants(), case_insensitive = true)]
+	bridge: RelayMessagesBridge,
+	/// Hex-encoded lane id that should be served by the relay. Defaults to `00000000`.
+	#[structopt(long, default_value = "00000000")]
+	lane: HexLaneId,
+	#[structopt(flatten)]
+	source: SourceConnectionParams,
+	#[structopt(flatten)]
+	source_sign: SourceSigningParams,
+	#[structopt(flatten)]
+	target: TargetConnectionParams,
+	#[structopt(flatten)]
+	target_sign: TargetSigningParams,
+	#[structopt(flatten)]
+	prometheus_params: PrometheusParams,
+}
+
+arg_enum! {
+	#[derive(Debug)]
+	/// Headers relay bridge.
+	pub enum RelayMessagesBridge {
+		MillauToRialto,
+		RialtoToMillau,
+	}
+}
+
+macro_rules! select_bridge {
+	($bridge: expr, $generic: tt) => {
+		match $bridge {
+			RelayMessagesBridge::MillauToRialto => {
+				type Source = relay_millau_client::Millau;
+				type Target = relay_rialto_client::Rialto;
+				use crate::rialto_millau::millau_messages_to_rialto::run;
+
+				$generic
+			}
+			RelayMessagesBridge::RialtoToMillau => {
+				type Source = relay_rialto_client::Rialto;
+				type Target = relay_millau_client::Millau;
+				use crate::rialto_millau::rialto_messages_to_millau::run;
+
+				$generic
+			}
+		}
+	};
+}
+
+impl RelayMessages {
+	/// Run the command.
+	pub async fn run(self) -> anyhow::Result<()> {
+		select_bridge!(self.bridge, {
+			let source_client = crate::rialto_millau::source_chain_client::<Source>(self.source).await?;
+			let source_sign =
+				Source::source_signing_params(self.source_sign).map_err(|e| anyhow::format_err!("{}", e))?;
+			let target_client = crate::rialto_millau::target_chain_client::<Target>(self.target).await?;
+			let target_sign =
+				Target::target_signing_params(self.target_sign).map_err(|e| anyhow::format_err!("{}", e))?;
+
+			run(
+				source_client,
+				source_sign,
+				target_client,
+				target_sign,
+				self.lane.into(),
+				self.prometheus_params.into(),
+			)
+			.await
+			.map_err(|e| anyhow::format_err!("{}", e))
+		})
+	}
+}
diff --git a/bridges/relays/bin-substrate/src/rialto_millau/cli.rs b/bridges/relays/bin-substrate/src/rialto_millau/cli.rs
index 1005e85472..3d314cc71b 100644
--- a/bridges/relays/bin-substrate/src/rialto_millau/cli.rs
+++ b/bridges/relays/bin-substrate/src/rialto_millau/cli.rs
@@ -1,4 +1,4 @@
-// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
 // This file is part of Parity Bridges Common.
 
 // Parity Bridges Common is free software: you can redistribute it and/or modify
@@ -20,57 +20,10 @@ use frame_support::weights::Weight;
 use structopt::StructOpt;
 
 use crate::cli::{
-	AccountId, Balance, ExplicitOrMaximal, HexBytes, HexLaneId, Origins, PrometheusParams, SourceConnectionParams,
-	SourceSigningParams, TargetConnectionParams, TargetSigningParams,
+	AccountId, Balance, ExplicitOrMaximal, HexBytes, HexLaneId, Origins, SourceConnectionParams, SourceSigningParams,
+	TargetSigningParams,
 };
 
-/// Start message relayer process.
-///
-/// TODO [#855] Move to separate module.
-#[derive(StructOpt)]
-pub enum RelayMessages {
-	/// Serve given lane of Millau -> Rialto messages.
-	MillauToRialto {
-		#[structopt(flatten)]
-		source: SourceConnectionParams,
-		#[structopt(flatten)]
-		source_sign: SourceSigningParams,
-		#[structopt(flatten)]
-		target: TargetConnectionParams,
-		#[structopt(flatten)]
-		target_sign: TargetSigningParams,
-		#[structopt(flatten)]
-		prometheus_params: PrometheusParams,
-		/// Hex-encoded lane id that should be served by the relay. Defaults to `00000000`.
-		#[structopt(long, default_value = "00000000")]
-		lane: HexLaneId,
-	},
-	/// Serve given lane of Rialto -> Millau messages.
-	RialtoToMillau {
-		#[structopt(flatten)]
-		source: SourceConnectionParams,
-		#[structopt(flatten)]
-		source_sign: SourceSigningParams,
-		#[structopt(flatten)]
-		target: TargetConnectionParams,
-		#[structopt(flatten)]
-		target_sign: TargetSigningParams,
-		#[structopt(flatten)]
-		prometheus_params: PrometheusParams,
-		/// Hex-encoded lane id that should be served by the relay. Defaults to `00000000`.
-		#[structopt(long, default_value = "00000000")]
-		lane: HexLaneId,
-	},
-}
-
-impl RelayMessages {
-	/// Run the command.
-	pub async fn run(self) -> anyhow::Result<()> {
-		super::run_relay_messages(self).await.map_err(format_err)?;
-		Ok(())
-	}
-}
-
 /// Send bridge message.
 ///
 /// TODO [#855] Move to separate module.
diff --git a/bridges/relays/bin-substrate/src/rialto_millau/mod.rs b/bridges/relays/bin-substrate/src/rialto_millau/mod.rs
index 7a48119be7..5446e382a7 100644
--- a/bridges/relays/bin-substrate/src/rialto_millau/mod.rs
+++ b/bridges/relays/bin-substrate/src/rialto_millau/mod.rs
@@ -1,4 +1,4 @@
-// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
 // This file is part of Parity Bridges Common.
 
 // Parity Bridges Common is free software: you can redistribute it and/or modify
@@ -44,63 +44,6 @@ use sp_runtime::{traits::IdentifyAccount, MultiSigner};
 use sp_version::RuntimeVersion;
 use std::fmt::Debug;
 
-async fn run_relay_messages(command: cli::RelayMessages) -> Result<(), String> {
-	match command {
-		cli::RelayMessages::MillauToRialto {
-			source,
-			source_sign,
-			target,
-			target_sign,
-			prometheus_params,
-			lane,
-		} => {
-			type Source = Millau;
-			type Target = Rialto;
-
-			let source_client = source_chain_client::<Source>(source).await?;
-			let source_sign = Source::source_signing_params(source_sign)?;
-			let target_client = target_chain_client::<Target>(target).await?;
-			let target_sign = Target::target_signing_params(target_sign)?;
-
-			millau_messages_to_rialto::run(
-				source_client,
-				source_sign,
-				target_client,
-				target_sign,
-				lane.into(),
-				prometheus_params.into(),
-			)
-			.await
-		}
-		cli::RelayMessages::RialtoToMillau {
-			source,
-			source_sign,
-			target,
-			target_sign,
-			prometheus_params,
-			lane,
-		} => {
-			type Source = Rialto;
-			type Target = Millau;
-
-			let source_client = source_chain_client::<Source>(source).await?;
-			let source_sign = Source::source_signing_params(source_sign)?;
-			let target_client = target_chain_client::<Target>(target).await?;
-			let target_sign = Target::target_signing_params(target_sign)?;
-
-			rialto_messages_to_millau::run(
-				source_client,
-				source_sign,
-				target_client,
-				target_sign,
-				lane.into(),
-				prometheus_params.into(),
-			)
-			.await
-		}
-	}
-}
-
 async fn run_send_message(command: cli::SendMessage) -> Result<(), String> {
 	match command {
 		cli::SendMessage::MillauToRialto {