diff --git a/crates/relay/src/api/proposer/error.rs b/crates/relay/src/api/proposer/error.rs
index 37533f34e..f775e7c79 100644
--- a/crates/relay/src/api/proposer/error.rs
+++ b/crates/relay/src/api/proposer/error.rs
@@ -41,6 +41,9 @@ pub enum ProposerApiError {
     #[error("proposer not registered")]
     ProposerNotRegistered,
 
+    #[error("unknown validators registration found")]
+    UnknownValidatorsRegistration,
+
     #[error("timestamp too early. {timestamp} < {min_timestamp}")]
     TimestampTooEarly { timestamp: u64, min_timestamp: u64 },
 
@@ -134,6 +137,7 @@ impl IntoResponse for ProposerApiError {
                 ProposerApiError::InvalidFork |
                 ProposerApiError::SerdeDecodeError(_) |
                 ProposerApiError::ProposerNotRegistered |
+                ProposerApiError::UnknownValidatorsRegistration |
                 ProposerApiError::TimestampTooEarly { .. } |
                 ProposerApiError::TimestampTooFarInTheFuture { .. } |
                 ProposerApiError::RequestWrongSlot { .. } |
diff --git a/crates/relay/src/api/proposer/register.rs b/crates/relay/src/api/proposer/register.rs
index 4e1cedb5f..db4adc4ef 100644
--- a/crates/relay/src/api/proposer/register.rs
+++ b/crates/relay/src/api/proposer/register.rs
@@ -144,6 +144,10 @@ impl<A: Api> ProposerApi<A> {
         REGISTRATIONS_UNKNOWN.inc_by(unknown_registrations);
         REGISTRATIONS_SKIPPED.inc_by(skipped_registrations);
 
+        if unknown_registrations > 0 {
+            return Err(ProposerApiError::UnknownValidatorsRegistration);
+        }
+
         if registrations_to_check.is_empty() {
             return Ok(StatusCode::OK);
         }