DETAILS of how British Intelligence suffered its most embarrassing setback since Iraq were revealed today, after an apparently senior Taliban commander promoted by MI6 as the key to an Afghan peace process was exposed as a lowly impostor.

An investigation by The (London) Times shows that British agents paid Mullah Akhtar Mohammad Mansour from May this year, promoting him as a genuine Taliban figure of the highest standing who was capable of negotiating with senior American and Afghan officials.

But according to officials in Britain, America and Afghanistan, he was uncovered this month as a fraudster, dealing a blow to the credibility of the Secret Intelligence Service, MI6.

Far from being a former Taliban government minister, the individual concerned is now thought to have been a shopkeeper, a minor Taliban commander, or simply a well-connected chancer from the Pakistani border city of Quetta.

A senior Afghan government official said yesterday, "British Intelligence was naive and there was wishful thinking on our part".

One source with knowledge of the affair described it as simply, "a major f**k-up".

Last night, Bill Harris, who retired this month as the most senior US representative in Kandahar province, told The Times that it was not British intelligence officers alone who were responsible for the error. "Something this stupid generally requires teamwork," he said.