He was seen on the Tube on drug detection duties with his sniffer dog.

Passengers were impressed as he produced an ID card and told them he was with MI5 working with police. Even Tube staff who questioned him thought him believable.

But it was all a lie.

Jason Marshall, 23, of Monega Road, Forest Gate, was a thief and a fraud. His ID was fake and his “sniffer dog” was his pet Scottish terrier.

On April 6, Marshall was seen by station staff at Dagenham Heathway conducting a drugs detection operation, Blackfriars Crown Court heard. When challenged he said he was working for MI5, but was on secondment to British Transport Police.

He produced a fake ID card and got access to an office where he stole a London Underground radio.

Two days later, Marshall boarded a District Line train at Elm Park station and began to check people’s tickets with an Oyster Card reader he had stolen from Barking station.

But a revenue inspector became suspicious. He took the card reader, ID and radio and at Upney alerted colleagues who arrested Marshall.

After admitting two counts of impersonating a police officer, one of handling stolen goods and one of possession of a false ID card with improper intention, Marshall was given an eight-month suspended prison sentence.

Det Con Graham Hempstead said: “Marshall was committing a serious offence by pretending to be a police officer and has rightly been punished.”