Sacked Foreign Office chief Sir Olly Robbins is set for an explosive clash with MPs on Tuesday which could make or break the Prime Minister.

One key ally said Sir Olly, appointed as the department's permanent under-secretary by Sir Keir Starmer in January 2025, is furious at the way he has been treated over the Peter Mandelson controversy.

Sir Olly has been accused of not telling Downing Street that Mandelson had failed his UK security vetting after he was appointed US ambassador.

He had yet to formally accept the foreign affairs committee's invitation last night, but it is expected that he will appear to launch a passionate defence of his actions.

Sir Olly – who took up his role after Mandelson had been appointed but before his vetting – was sacked on Thursday night, hours after news broke of the Labour grandee's clearance failure.

Downing Street insists it learned the truth only two days before it was revealed by The Guardian. 

Sir Olly is adamant he followed protocol when overruling the vetting decision and in not telling No 10.

Serious concerns were raised about Mandelson by the Government's specialist vetting agency following an investigation into his finances, personal relationships and foreign contacts, and officials recommended he should not be granted clearance.

It fell to Sir Olly – only a fortnight into his role as the Foreign Office's most senior civil servant and dealing with an appointment Sir Keir had already publicly announced before vetting had even begun – to make the final call, and he waved him through.

Sir Keir is promising to come clean in a statement to the Commons tomorrow.

But opponents will then pore over every word from Sir Olly that points to any suggestions the Prime Minister – who repeatedly assured MPs 'due process' had been followed – has knowing misled them, or acted unfairly towards Sir Olly. 

The former Foreign Office chief is understood to believe vehemently that it would have been a breach of his obligations had he informed Sir Keir about Mandelson's vetting failure.

Among those coming out in support of him yesterday was Lord McDonald, also a former permanent under-secretary at the Foreign Office.

He accused Sir Keir of sacking Sir Olly because he wanted 'a scalp as quickly as possible within the news cycle'.

Lord McDonald said: 'I cannot see that there was any process, any fairness, any giving him the chance to set out his case – and that feels to me wrong.' 

Asked on BBC Radio 4's Today programme whether he 'bought' the Government's claims about not knowing Mandelson had failed the vetting, Lord McDonald replied: 'No, I do not.'

He added: 'If there had been a failure, then that fact would have to be conveyed to the political level, but the fact that it was not indicates to me that the picture was more complicated than No 10 wishes to present.'

A source on the foreign affairs committee told The Mail on Sunday that Sir Olly will be 'dragged over the coals' on crucial questions of exactly who knew what and when. 

He will be called on to explain the truth about whether he alone decided to overrule Mandelson's vetting failure – as Sir Keir claims – or whether he spoke to anyone else in the Foreign Office or Downing Street.

The role of both David Lammy and Yvette Cooper, the former and serving Foreign Secretaries, will also be examined.

MPs on the committee will want to know whether Ms Cooper asked about Mandelson's vetting status following the release of the Jeffrey Epstein files by the US government, which revealed a close friendship between the pair.

The committee source said that any failure by key players to ask Sir Olly about Mandelson's vetting after the publication of the files would have been 'a gross dereliction of duty'.

MPs will want to get to the bottom of why Sir Olly lost his job for failing to tell the Prime Minister about the vetting outcome – and will compare his treatment to that of Cabinet Secretary Dame Antonia Romeo.

It was revealed on Friday that Dame Antonia knew about Mandelson's vetting failure for more than a fortnight before the Prime Minister claims to have found out.

It is understood he is standing by his latest Cabinet Secretary, who was only appointed in February.