MI5 agent 'resigns over sting'

The man's wife is believed to be a prostitute who handed the News of the World evidence that Mr Mosley had taken part in a sado-masochist orgy.

The disclosure will raise questions over vetting procedures in Britain's security service.

However, MI5 director general Jonathan Evans has reportedly given Gordon Brown and Home Secretary Jacqui Smith a categorical assurance that the organisation itself was not involved in the sting.

According to the Sunday Telegraph, the agent at the centre of the controversy was in his 40s and had been working as a surveillance operative for several years.

MI5 vetting should have entailed regular questioning about his private life, covering details of relationships, sexual habits, and intake of drugs and alcohol.

The system is designed to detect whether individuals are susceptible to blackmail, and therefore pose a security risk. However, it seems important information slipped through the net on this occasion.

The agent apparently resigned immediately when confronted with evidence about the involvement of his wife - who is thought to have been paid tens of thousands of pounds for video footage.

The News of the World published its story about Mr Mosley at the end of March, claiming he paid £2,500 for an orgy with five prostitutes that had a "sick" Nazi theme.

But 68-year-old Mr Mosley, president of motor racing body the FIA, has denied there were any Nazi or racist overtones. He is seeking damages for invasion of privacy.