The former Ministry of Defence (MoD) director general’s blunder led to a 50-page dossier being found in a “soggy heap” at a bus stop in Kent in June, prompting both fears of espionage by “adversaries” and ridicule of the UK’s intelligence and security establishment.

The documents – one of which was rated at the highest ‘Secret: UK Eyes Only’ classification – went public after a passerby handed them to the BBC, which published a report detailing some of their contents. Last month, an MoD probe had pinned the blame on a single public servant but did not identify the individual.

On Tuesday, The Guardian cited two anonymous government sources to reveal that Lapsley – who was director general strategy & international at the time of the incident – was responsible.

Despite the gaffe, the paper reported that his appointment as NATO ambassador was now “unlikely, but not definitely ruled out”. In his MoD position, he was responsible for “defence policy on NATO and the general Euro-Atlantic area.”

However, Lapsley’s security clearance has been revoked pending a review and he has been transferred to the Foreign, Commonwealth and Development Office.

But an unnamed government source told The Telegraph that Lapsley’s security clearance could be “reinstated at a later date”.

Although a final punishment is pending, The Guardian claimed the most likely penalty would be an extended suspension of Lapsley’s clearance to see classified information, which is apparently likely to last a few months instead of weeks.

Security officials in both the UK and the US have been critical of Westminster’s response to the leak and news about Lapsley’s NATO chances.

“It used to be the case that people would be hung out to dry for something like this,” security sources reportedly told The Guardian, adding that the lack of a strong official reprimand would make it harder for government departments to penalise public servants responsible for similar slip-ups in the future.

Meanwhile, senior US Defense Department sources told The Telegraph that “an FBI investigation would ensue” if the situation had played out in the US and warned that the “lack of discipline” taken against Lapsley raised “serious questions about UK-US intelligence”.

"At the very least, the person in question would either be asked to resign, forced to take early retirement or be permanently stripped of his security clearance and redeployed to a role where no security clearance was required."

According to the BBC, the document marked ‘Secret: UK Eyes Only’ had sensitive recommendations for future UK military deployment in Afghanistan once US and NATO operations wound up.

These top-secret documents are printed on pink paper and are not supposed to be taken out of government buildings unless properly logged out and securely stored.

The dossier also revealed high-level British military deliberations about the possible Russian reactions to the HMS ‘Defender’ warship's passage by the coast of Crimea.

The mission – dubbed ‘Op Ditroite’ – ended with Russian forces firing warning shots at the vessel on June 23 after it ignored demands to leave waters off Cape Fiolent, near the Crimean city of Sevastopol.

Last month, UK Defence Minister Ben Wallace told Parliament that an official investigation into the incident had turned up “no evidence of espionage” and concluded that there had been “no compromise of the papers by our adversaries”.

A government spokesman echoed those comments to The Telegraph, but US officials told the paper that “the way [the incident] has been handled does not instill confidence.”

“Why was he even taking such documents out of the building? Obviously, the British are more lax than we are,” they said.