diff --git a/substrate/frame/beefy/src/default_weights.rs b/substrate/frame/beefy/src/default_weights.rs
index 70dd3bb02bf1e..6b83015459d61 100644
--- a/substrate/frame/beefy/src/default_weights.rs
+++ b/substrate/frame/beefy/src/default_weights.rs
@@ -57,11 +57,6 @@ impl crate::WeightInfo for () {
 			.saturating_add(DbWeight::get().reads(2))
 	}
 
-	// TODO: Calculate
-	fn report_fork_voting(_validator_count: u32, _max_nominators_per_validator: u32) -> Weight {
-		Weight::MAX
-	}
-
 	fn set_new_genesis() -> Weight {
 		DbWeight::get().writes(1)
 	}
diff --git a/substrate/frame/beefy/src/lib.rs b/substrate/frame/beefy/src/lib.rs
index ab1a7df961e6a..821e7c939b551 100644
--- a/substrate/frame/beefy/src/lib.rs
+++ b/substrate/frame/beefy/src/lib.rs
@@ -294,9 +294,10 @@ pub mod pallet {
 		/// and validate the given key ownership proof against the extracted offender.
 		/// If both are valid, the offence will be reported.
 		#[pallet::call_index(3)]
-		#[pallet::weight(T::WeightInfo::report_fork_voting(
+		#[pallet::weight(T::WeightInfo::report_fork_voting::<T>(
 			key_owner_proof.validator_count(),
 			T::MaxNominators::get(),
+			&equivocation_proof.ancestry_proof
 		))]
 		pub fn report_fork_voting(
 			origin: OriginFor<T>,
@@ -328,9 +329,10 @@ pub mod pallet {
 		/// if the block author is defined it will be defined as the equivocation
 		/// reporter.
 		#[pallet::call_index(4)]
-		#[pallet::weight(T::WeightInfo::report_fork_voting(
+		#[pallet::weight(T::WeightInfo::report_fork_voting::<T>(
 			key_owner_proof.validator_count(),
 			T::MaxNominators::get(),
+			&equivocation_proof.ancestry_proof
 		))]
 		pub fn report_fork_voting_unsigned(
 			origin: OriginFor<T>,
@@ -740,11 +742,36 @@ pub trait WeightInfo {
 		max_nominators_per_validator: u32,
 	) -> Weight;
 
+	fn set_new_genesis() -> Weight;
+}
+
+pub(crate) trait WeightInfoExt: WeightInfo {
 	fn report_double_voting(validator_count: u32, max_nominators_per_validator: u32) -> Weight {
 		Self::report_voting_equivocation(2, validator_count, max_nominators_per_validator)
 	}
 
-	fn report_fork_voting(validator_count: u32, max_nominators_per_validator: u32) -> Weight;
+	fn report_fork_voting<T: Config>(
+		validator_count: u32,
+		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()
+			.saturating_add(
+				<T::AncestryHelper as AncestryHelperWeightInfo<HeaderFor<T>>>::is_non_canonical(
+					ancestry_proof,
+				),
+			)
+			.saturating_add(Self::report_voting_equivocation(
+				1,
+				validator_count,
+				max_nominators_per_validator,
+			));
+
+		// TODO: 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(
 		validator_count: u32,
@@ -760,6 +787,6 @@ pub trait WeightInfo {
 				max_nominators_per_validator,
 			))
 	}
-
-	fn set_new_genesis() -> Weight;
 }
+
+impl<T> WeightInfoExt for T where T: WeightInfo {}
diff --git a/substrate/frame/beefy/src/tests.rs b/substrate/frame/beefy/src/tests.rs
index a63b3532b6983..d75237205cac2 100644
--- a/substrate/frame/beefy/src/tests.rs
+++ b/substrate/frame/beefy/src/tests.rs
@@ -35,7 +35,7 @@ use sp_consensus_beefy::{
 use sp_runtime::DigestItem;
 use sp_session::MembershipProof;
 
-use crate::{self as beefy, mock::*, Call, Config, Error, WeightInfo};
+use crate::{self as beefy, mock::*, Call, Config, Error, WeightInfoExt};
 
 fn init_block(block: u64) {
 	System::set_block_number(block);
@@ -765,7 +765,9 @@ fn report_double_voting_has_valid_weight() {
 	// the weight depends on the size of the validator set,
 	// but there's a lower bound of 100 validators.
 	assert!((1..=100)
-		.map(|validators| <Test as Config>::WeightInfo::report_double_voting(validators, 1000))
+		.map(|validators| <<Test as Config>::WeightInfo as WeightInfoExt>::report_double_voting(
+			validators, 1000
+		))
 		.collect::<Vec<_>>()
 		.windows(2)
 		.all(|w| w[0] == w[1]));
@@ -773,7 +775,9 @@ fn report_double_voting_has_valid_weight() {
 	// after 100 validators the weight should keep increasing
 	// with every extra validator.
 	assert!((100..=1000)
-		.map(|validators| <Test as Config>::WeightInfo::report_double_voting(validators, 1000))
+		.map(|validators| <<Test as Config>::WeightInfo as WeightInfoExt>::report_double_voting(
+			validators, 1000
+		))
 		.collect::<Vec<_>>()
 		.windows(2)
 		.all(|w| w[0].ref_time() < w[1].ref_time()));