diff --git a/polkadot/runtime/rococo/src/weights/pallet_beefy_mmr.rs b/polkadot/runtime/rococo/src/weights/pallet_beefy_mmr.rs
index 317c9149ec6c5..8371d63383b20 100644
--- a/polkadot/runtime/rococo/src/weights/pallet_beefy_mmr.rs
+++ b/polkadot/runtime/rococo/src/weights/pallet_beefy_mmr.rs
@@ -48,6 +48,10 @@ use core::marker::PhantomData;
 /// Weight functions for `pallet_beefy_mmr`.
 pub struct WeightInfo<T>(PhantomData<T>);
 impl<T: frame_system::Config> pallet_beefy_mmr::WeightInfo for WeightInfo<T> {
+	fn n_leafs_proof_is_optimal(n: u32) -> Weight {
+		todo!()
+	}
+
 	/// Storage: `System::BlockHash` (r:1 w:0)
 	/// Proof: `System::BlockHash` (`max_values`: None, `max_size`: Some(44), added: 2519, mode: `MaxEncodedLen`)
 	fn extract_validation_context() -> Weight {
diff --git a/polkadot/runtime/westend/src/weights/pallet_beefy_mmr.rs b/polkadot/runtime/westend/src/weights/pallet_beefy_mmr.rs
index 5be207e3fcff4..157349038c314 100644
--- a/polkadot/runtime/westend/src/weights/pallet_beefy_mmr.rs
+++ b/polkadot/runtime/westend/src/weights/pallet_beefy_mmr.rs
@@ -48,6 +48,10 @@ use core::marker::PhantomData;
 /// Weight functions for `pallet_beefy_mmr`.
 pub struct WeightInfo<T>(PhantomData<T>);
 impl<T: frame_system::Config> pallet_beefy_mmr::WeightInfo for WeightInfo<T> {
+	fn n_leafs_proof_is_optimal(n: u32) -> Weight {
+		todo!()
+	}
+
 	/// Storage: `System::BlockHash` (r:1 w:0)
 	/// Proof: `System::BlockHash` (`max_values`: None, `max_size`: Some(44), added: 2519, mode: `MaxEncodedLen`)
 	fn extract_validation_context() -> Weight {
diff --git a/substrate/frame/beefy-mmr/src/benchmarking.rs b/substrate/frame/beefy-mmr/src/benchmarking.rs
index fea6a2078f0f1..4fddd1bccf115 100644
--- a/substrate/frame/beefy-mmr/src/benchmarking.rs
+++ b/substrate/frame/beefy-mmr/src/benchmarking.rs
@@ -49,6 +49,24 @@ fn init_block<T: Config>(block_num: u32) {
 mod benchmarks {
 	use super::*;
 
+	/// Generate ancestry proofs with `n` leafs and benchmark the logic that checks
+	/// if the proof is optimal.
+	#[benchmark]
+	fn n_leafs_proof_is_optimal(n: Linear<2, 512>) {
+		pallet_mmr::UseLocalStorage::<T>::set(true);
+
+		for block_num in 1..=n {
+			init_block::<T>(block_num);
+		}
+		let proof = Mmr::<T>::generate_mock_ancestry_proof().unwrap();
+		assert_eq!(proof.leaf_count, n as u64);
+
+		#[block]
+		{
+			<BeefyMmr<T> as AncestryHelper<HeaderFor<T>>>::is_proof_optimal(&proof);
+		};
+	}
+
 	#[benchmark]
 	fn extract_validation_context() {
 		pallet_mmr::UseLocalStorage::<T>::set(true);
diff --git a/substrate/frame/beefy-mmr/src/lib.rs b/substrate/frame/beefy-mmr/src/lib.rs
index ebdc9db06a62e..c7fcdeff87999 100644
--- a/substrate/frame/beefy-mmr/src/lib.rs
+++ b/substrate/frame/beefy-mmr/src/lib.rs
@@ -304,6 +304,10 @@ impl<T: Config> AncestryHelperWeightInfo<HeaderFor<T>> for Pallet<T>
 where
 	T: pallet_mmr::Config<Hashing = sp_consensus_beefy::MmrHashing>,
 {
+	fn is_proof_optimal(proof: &<Self as AncestryHelper<HeaderFor<T>>>::Proof) -> Weight {
+		<T as Config>::WeightInfo::n_leafs_proof_is_optimal(proof.leaf_count.saturated_into())
+	}
+
 	fn extract_validation_context() -> Weight {
 		<T as Config>::WeightInfo::extract_validation_context()
 	}
diff --git a/substrate/frame/beefy-mmr/src/weights.rs b/substrate/frame/beefy-mmr/src/weights.rs
index dcfdb560ee949..fe275b4fd3753 100644
--- a/substrate/frame/beefy-mmr/src/weights.rs
+++ b/substrate/frame/beefy-mmr/src/weights.rs
@@ -51,6 +51,7 @@ use core::marker::PhantomData;
 
 /// Weight functions needed for `pallet_beefy_mmr`.
 pub trait WeightInfo {
+	fn n_leafs_proof_is_optimal(n: u32, ) -> Weight;
 	fn extract_validation_context() -> Weight;
 	fn read_peak() -> Weight;
 	fn n_items_proof_is_non_canonical(n: u32, ) -> Weight;
@@ -59,6 +60,10 @@ pub trait WeightInfo {
 /// Weights for `pallet_beefy_mmr` using the Substrate node and recommended hardware.
 pub struct SubstrateWeight<T>(PhantomData<T>);
 impl<T: frame_system::Config> WeightInfo for SubstrateWeight<T> {
+	fn n_leafs_proof_is_optimal(n: u32) -> Weight {
+		todo!()
+	}
+
 	/// Storage: `System::BlockHash` (r:1 w:0)
 	/// Proof: `System::BlockHash` (`max_values`: None, `max_size`: Some(44), added: 2519, mode: `MaxEncodedLen`)
 	fn extract_validation_context() -> Weight {
@@ -98,6 +103,10 @@ impl<T: frame_system::Config> WeightInfo for SubstrateWeight<T> {
 
 // For backwards compatibility and tests.
 impl WeightInfo for () {
+	fn n_leafs_proof_is_optimal(n: u32) -> Weight {
+		todo!()
+	}
+
 	/// Storage: `System::BlockHash` (r:1 w:0)
 	/// Proof: `System::BlockHash` (`max_values`: None, `max_size`: Some(44), added: 2519, mode: `MaxEncodedLen`)
 	fn extract_validation_context() -> Weight {
diff --git a/substrate/frame/beefy/src/lib.rs b/substrate/frame/beefy/src/lib.rs
index cf690a9df339d..e57fc0e21bc1e 100644
--- a/substrate/frame/beefy/src/lib.rs
+++ b/substrate/frame/beefy/src/lib.rs
@@ -755,7 +755,8 @@ pub(crate) trait WeightInfoExt: WeightInfo {
 		max_nominators_per_validator: u32,
 		ancestry_proof: &<T::AncestryHelper as AncestryHelper<HeaderFor<T>>>::Proof,
 	) -> Weight {
-		let _weight = <T::AncestryHelper as AncestryHelperWeightInfo<HeaderFor<T>>>::extract_validation_context()
+		<T::AncestryHelper as AncestryHelperWeightInfo<HeaderFor<T>>>::is_proof_optimal(&ancestry_proof)
+			.saturating_add(<T::AncestryHelper as AncestryHelperWeightInfo<HeaderFor<T>>>::extract_validation_context())
 			.saturating_add(
 				<T::AncestryHelper as AncestryHelperWeightInfo<HeaderFor<T>>>::is_non_canonical(
 					ancestry_proof,
@@ -765,12 +766,7 @@ pub(crate) trait WeightInfoExt: WeightInfo {
 				1,
 				validator_count,
 				max_nominators_per_validator,
-			));
-
-		// TODO: https://github.com/paritytech/polkadot-sdk/issues/4523 - return `_weight` here.
-		// We return `Weight::MAX` currently in order to disallow this extrinsic for the moment.
-		// We need to check that the proof is optimal.
-		Weight::MAX
+			))
 	}
 
 	fn report_future_block_voting(
diff --git a/substrate/frame/beefy/src/mock.rs b/substrate/frame/beefy/src/mock.rs
index 26ce472b9c82f..476777ab1e57a 100644
--- a/substrate/frame/beefy/src/mock.rs
+++ b/substrate/frame/beefy/src/mock.rs
@@ -147,6 +147,10 @@ impl<Header: HeaderT> AncestryHelper<Header> for MockAncestryHelper {
 }
 
 impl<Header: HeaderT> AncestryHelperWeightInfo<Header> for MockAncestryHelper {
+	fn is_proof_optimal(_proof: &<Self as AncestryHelper<HeaderFor<Test>>>::Proof) -> Weight {
+		unimplemented!()
+	}
+
 	fn extract_validation_context() -> Weight {
 		unimplemented!()
 	}
diff --git a/substrate/primitives/consensus/beefy/src/lib.rs b/substrate/primitives/consensus/beefy/src/lib.rs
index 4e358d30c3075..0f57cdfc81042 100644
--- a/substrate/primitives/consensus/beefy/src/lib.rs
+++ b/substrate/primitives/consensus/beefy/src/lib.rs
@@ -466,6 +466,9 @@ pub trait AncestryHelper<Header: HeaderT> {
 
 /// Weight information for the logic in `AncestryHelper`.
 pub trait AncestryHelperWeightInfo<Header: HeaderT>: AncestryHelper<Header> {
+	/// Weight info for the `AncestryHelper::is_proof_optimal()` method.
+	fn is_proof_optimal(proof: &<Self as AncestryHelper<Header>>::Proof) -> Weight;
+
 	/// Weight info for the `AncestryHelper::extract_validation_context()` method.
 	fn extract_validation_context() -> Weight;