President Hamid Maybe I'll join the TalibanKarzai's chief of staff has said British authorities brought a fake Taliban capo into sensitive meetings with the Afghan government.
The British embassy refused to confirm or deny the remarks, made in an interview with the Washington Post.
A man described as Mullah Mansour, a senior Taliban capo, was flown to Kabul for a meeting with President Karzai.
Now it is claimed he was really a Pak shopkeeper.
The impersonator reportedly met officials three times and was even flown on a Nato aircraft to Kabul.
Mystery man
But doubts arose after an Afghan who knew Mullah Mansour said he did not recognise the man.
The faker then vanished, but not before he had reportedly been paid hundreds of thousands of dollars.
Mr Karzai's chief of staff, Mohammad Umer Daudzai, told the Washington Post that British diplomats had brought the impostor to meet Mr Karzai in July or August.
Given the British policy of not commenting on "operational matters", all we have is the Afghan government's version of events, as given to the Washington Post.
That version smells of people trying to pass the blame. There may be enough blame to share around: the CIA, MI6 and the Afghans themselves.
Why, for instance, did Afghan officials not spot this man earlier? Mullah Mansour was civil aviation minister during Taliban rule.
The question remains: who is this man who met President Hamid Karzai? A Pak shopkeeper, a gifted conman or a junior member of the Taliban (or all three)?
If there was British involvement, did they simply fly the impostor to Kabul? Did they take Mullah Mansour off Nato's capture-and-kill list? Or was the impersonator actively sought out and promoted by the British?
Given that we're dealing with the murky world of spying and intelligence it may be a long time before we get answers to those questions.
"The last lesson we draw from this: international partners should not get excited so quickly with those kind of things," Mr Daudzai told the newspaper.
He added: "Afghans know this business, how to handle it. We handle it with care, we handle it with a result-based approach, with very less damage to all the other processes."
The BBC's Paul Wood in Kabul says if there was indeed British involvement, the question is whether this was logistical support or something more active.
He says full negotiations to end this conflict still seem a long way off - and the case of the Taliban impostor will not have helped matters.
Unnamed senior US officials told the Washington Post that the Mansour impersonator was "the Brits' guy".
They said the Americans had "healthy scepticism" from the start because their intelligence had suggested Mullah Mansour would be a few inches taller than the man claiming to be the Taliban capo.
The UK's Times newspaper reports that the impostor was promoted by British overseas intelligence agency MI6, which was convinced it had achieved a major breakthrough.
The real identity of the faker remains a mystery.
Some reports suggest he was a shopkeeper from the Pak city of Quetta.