One of the most baffling aspects of the Peter Mandelson vetting row has been the Prime Minister's willingness to alienate the dangerously silky mandarins of the Foreign Office.

Sacking Sir Olly Robbins as permanent secretary – on the grounds that Sir Olly should have told him that Mandelson had failed the vetting to be ambassador to the US – has pitted him against the might of the machine in Whitehall and around the world.

Surely, say senior Whitehall figures, he should have been stopped from making such a self-lacerating move by the most powerful civil servant in the country – Cabinet Secretary Dame Antonia Romeo.

It is not known what advice he was given, but the party-loving dame is herself said to be no fan of the Foreign Office, following a bitter Whitehall battle over claims she 'bullied' staff and misused expenses during a controversial spell serving at the British consulate in New York.

When Dame Antonia, 51, took up her position as head of the civil service in February, criticism of the appointment was led by Simon McDonald, who was her permanent secretary during her American sojourn.

Lord McDonald said there should be 'more due diligence' over the decision – civil service speak for 'don't give her the job'.

It is the same Lord McDonald who has led the Establishment's defence of Sir Olly during l'affaire Mandelson, saying he had been 'thrown under the bus' by No 10.

Dame Antonia's use of taxpayer-funded expenses and her alleged 'bullying' of staff while working as Her Majesty's Consul General in New York in 2016 led to a formal investigation, which found she had a 'case to answer' over claims that she 'terrorised' staff who criticised her lifestyle.

Her expenses included more than £120,000 in fees for her three children to attend an upmarket New York school and business-class flights – including a last-minute trip to London in February 2017 so Dame Antonia could attend the Baftas. She also hosted a series of glitzy parties for celebrity figures including Anna Wintour, Joanna Lumley and the now disgraced movie mogul Harvey Weinstein.

It has been reported that the Foreign Office's investigation concluded there was a case for Dame Antonia to answer about her behaviour towards colleagues, although no case to answer about 'financial probity'.

To the fury of Lord McDonald, the investigation was quashed by Whitehall officials. Then, in 2022, members of the Cabinet Office's propriety and ethics team broke into a Whitehall safe to obtain the department's copy of the investigation and 'personnel matters' relating to Dame Antonia: the papers were later destroyed.

The department insists that the safe was 'manually opened after multiple unsuccessful attempts to open it', that the action 'had nothing to do with any HR report or the Cabinet Secretary' and that the disposal of the papers was routine practice. Members of the Foreign Office who were serving at that time have told the Mail that they regard the whole process as 'a disgrace'.

The Guardian revealed last week that Mandelson was given the Washington job despite failing his developed vetting (DV) security clearance after multiple 'red flags' had been raised in Whitehall over his friendship with paedophile Jeffrey Epstein and his business links to Russia and China.

One senior Whitehall source suggested that Dame Antonia should have done more to protect Sir Keir from his own actions –thoughts echoed by Chancellor Rachel Reeves around the Cabinet table this week after four former Cabinet secretaries followed Lord McDonald in criticising Sir Olly's sacking.

The source said: 'I think Antonia has f***ed up. I think it is bizarre that No 10 have picked this battle with the Foreign Office when there is more stuff due to come out from the files which will be even more difficult for them.

'The extent to which we have discussed vetting, and the fact that the word "STRAP" [the top security clearance given to Mandelson to allow him to see the most secret government papers] is now on the front pages is hugely damaging to national security.'

The Government announced on Tuesday that it had launched an inquiry into the leak of the story.

Sir Olly told the foreign affairs committee earlier this week: 'I'm not making accusations at anybody, it's not my business to do so. I hope they're being very rigorously investigated and that prosecutions will result, because this is a grievous breach of national security.'

The Mail reported earlier this year that Dame Antonia had been handed the plum post because she would be more likely than her ousted predecessor as Cabinet Secretary, Sir Chris Wormald, to help to suppress sensitive documents relating to Mandelson.

Sources said the removal of Sir Chris was driven by Sir Keir's desire to keep a 'tight grip' on the private messages released in relation to Mandelson's appointment, and avoid exactly the sort of political conflagration which has now occurred.

MPs forced the Government to release the documents after a criminal investigation was launched into claims Mandelson passed sensitive information to Epstein. Dame Antonia, they said, was more prepared to cooperate with the Prime Minister's wish to restrict the release of the information as much as possible.

The MPs' motion made an exception for papers prejudicial to national security or international relations, which it said should be released to the intelligence and security committee.

The sources say Sir Chris was removed because he was taking a too 'liberal' interpretation of what files could be released.

A Cabinet Office spokesman said of the bullying investigation: 'The complaint was thoroughly investigated and all the allegations were dismissed on the basis there was no case to answer.'