A Question of Proof is a 1935 detective novel by Cecil Day-Lewis, written under the pen name of Nicholas Blake.

Consequently, the publishers Collins advertised the book as being written by a "well-known writer" using a pen name.

[3] It was a commercial success selling around 200,000 copies in Britain and launching Day-Lewis, who quickly did become widely identified as the author, as one of the leading writers of the Golden Age of Detective Fiction.

When the police's suspicions fall on teacher Michael Evans who has been having an affair with the headmaster's wife, he asks his old friend from Oxford Nigel Strangeways to investigate the real culprit.

He has his suspicions, but requires the necessary proof until a second murder is committed during a cricket match.