diff --git a/src/Application/Features/Candidates/Queries/Search/CandidateSearchQueryValidator.cs b/src/Application/Features/Candidates/Queries/Search/CandidateSearchQueryValidator.cs
index b37e9d96..618acd33 100644
--- a/src/Application/Features/Candidates/Queries/Search/CandidateSearchQueryValidator.cs
+++ b/src/Application/Features/Candidates/Queries/Search/CandidateSearchQueryValidator.cs
@@ -28,10 +28,11 @@ public CandidateSearchQueryValidator()
 
         RuleFor(q => q.ExternalIdentifier)
             .NotEmpty()
-            .NotNull()
             .WithMessage("External identifier is required")
             .Matches(ValidationConstants.AlphaNumeric)
-            .WithMessage(string.Format(ValidationConstants.AlphaNumericMessage, "External Identifier"));
+            .WithMessage(string.Format(ValidationConstants.AlphaNumericMessage, "External Identifier"))
+            .Length(7) // CRN and NOMIS Number (both 7 chars)
+            .WithMessage("Unrecognised format for External Identifier");
 
     }
 }
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