The senior civil servant who misplaced 50 pages of classified Ministry of Defence documents, which were later found at a bus stop in Kent, was being lined up to be appointed the UK’s ambassador to Nato at the time of the incident, according to two government sources.

The elevation of Angus Lapsley is now understood to be unlikely but not definitely ruled out in light of the unfortunate episode, in which the mislaid paperwork – some of which was marked secret – discussed sensitive deployments in Afghanistan and the Black Sea.

It became public because the paperwork was handed to the BBC at the end of June, prompting the broadcaster to put together a report detailing some of its contents.

Lapsley, who has not previously been named, has already had his security clearance suspended pending a full review and has been redeployed from the MoD to the Foreign, Commonwealth and Development Office, from where he was on secondment.

Without security clearance he could not continue in his MoD position, acting as a director general, responsible for defence policy on Nato and the general Euro-Atlantic area. But a final penalty has yet to be decided.

The lack of a definitive public sanction and talk of the Nato job has left some security sources in government unhappy. One argued it would make it harder for the MoD or other government departments to firmly penalise junior employees for making a similar mistake. “It used to be the case that people would be hung out to dry for something like this,” they added.

One of the documents was, the BBC said, marked “Secret UK Eyes Only”, and included sensitive recommendations for the UK’s future military footprint in Afghanistan, including any role for its special forces, once US and Nato operations finally wind up over the summer.

Such secret documents, printed out on pink paper, are not supposed to be taken from government buildings unless they are properly logged out and securely stored. A special case was used to store them when they were retrieved from the BBC, a Whitehall source said. The source added: “The documents should not have been taken out of the building in this way and in this case.”

Last month, James Sunderland, a former soldier and backbench Conservative MP, said the person who removed the documents “must be held fully to account” because “the incident must have involved the deliberate removal of pink – secret – documents from the MOD secure area”.

Last week, the MoD said in a written statement to parliament that “there was no evidence of espionage” and concluded that all the classified material was recovered after the leak to the BBC. There “has been no compromise of the papers by our adversaries”, the ministry added.

The abandoned papers also revealed that there were two possible routes under consideration for the HMS Defender in its recent voyage across the Black Sea, one briefly passing through the territorial waters of Russian-occupied Crimea and the other sailing many miles away.

It confirmed that the decision to sail the warship close to Crimea and provoke Russia was a deliberate choice by the UK.

Lapsley is a respected long-serving official, who first became a civil servant in 1991 and acted as a private secretary for Tony Blair in the early phase of his premiership before moving on to the Foreign and Commonwealth Office.

The civil servant regularly features on thinktank panels discussing defence and foreign policy issues, and deputised for the defence secretary, Ben Wallace, at the Munich Security Conference in February 2020.

Last week the MoD said it would not make any comment on the identity of the individual who mislaid the documents, citing security reasons. But government sources told the Guardian there was no security reason not to name Lapsley.

The most likely punishment the civil servant will face is the further suspension of his clearance to see classified material, which is likely to last months rather than weeks.

A Foreign Office spokesperson said the UK took the protection of classified information seriously. “The individual concerned has been removed from sensitive work and has already had their security clearance suspended pending a full review,” they added.