The employee in Moncrieffe v London Underground Limited, a train driver, was dismissed after an investigation found that he had falsely claimed that a wallet which had been found in the driver's cab of a train left in a siding belonged to him. The evidence was that a cleaner had found the wallet, had notified one of the drivers who had asked the claimant if it was his. The claimant said it was and so the driver gave it to him. Later a third driver claimed that the wallet belonged to him.

The claim for unfair dismissal was rejected by the Tribunal on the basis that the employer had reasonable grounds for rejecting the claimant's account that he believed it was his wallet. The claimant appealed, arguing that the investigation was not sufficiently extensive, given the grave allegation, the potentially very serious consequences for him, and his long unblemished record. He maintained that, as there was an issue of whether he had been on the train in question, his manager should have been interviewed and that the driver who had claimed to own the wallet should have been interviewed in depth to deal with discrepancies in the various descriptions of the wallet.

The EAT found that despite his 14 years' unblemished service, and the employer's failure to interview two potential witnesses, the Tribunal had not been wrong to conclude that there had been a reasonable investigation.

The EAT did, however, comment on the principle set out in A v B and Salford Royal NHS Foundation Trust v Roldan, that the gravity of the consequences for the employee have to be taken into account when assessing the reasonableness of an investigation. This principle has been applied particularly in regulated employment, where dismissal may result in the employee being disqualified from their profession. In Moncrieffe the EAT judge expressed his "forthright" opinion that the test should apply regardless of the nature of the work carried out by the employee.