Sir Olly Robbins - the recently sacked Foreign Office chief - is set to make a bombshell accusation against Sir Keir Starmer to MPs today.
His appearance before the Foreign Affairs Select Committee has been touted as "box office", with one senior Whitehall source telling The Times he will "not hold back".
Sir Olly is expected to accuse the PM of pressuring the Foreign Office into approving Lord Mandelson's appointment - despite knowing about his friendship with the paedophile Jeffrey Epstein and his links to Russia and China.
He was sacked from his position last week, with Sir Keir accusing the senior civil servant of a cover-up and saying the decision not to inform him that Lord Mandelson's failed security vetting "beggars belief".
But now, Sir Olly will use his appearance this morning to reveal that he did not see the advice from UK Security Vetting (UKSV), which recommended against the appointment.
He was given a "verbal briefing" by the Foreign Office, and was told that UKSV considered Lord Mandelson's case to be "borderline" - though if it was up to UKSV it was unlikely he would have been given the green light.
Sir Olly then assessed the "outstanding risks" and concluded that they could be mitigated, he is set to say.
His "overarching narrative", meanwhile, is that the vetting process is confidential and shielded from ministers.
Ahead of the ousted civil servant's appearance, Sir Keir told MPs yesterday: “A deliberate decision was taken to withhold that material. There was not a lack of asking. This wasn’t an oversight. It was a decision. It was a decision taken not to share that information on repeated occasions.”
FOLLOW BELOW FOR LIVE UPDATES THROUGHOUT THE DAY...
'The Prime Minister should go' - Shadow Housing Secretary calls Peter Mandelson scandal 'unbelievable'
Sir James Cleverly, Shadow Secretary for Housing, said, that the Government's claim that they didn't ask on the appointment process made on Lord Peter Mandelson is "unbelievable".
Speaking to GB News, he said: "I would have asked, I would of said, 'what's happened to the vetting? Has he got the green light? I need to know'."
Sir James continued to say that Lord Mandelson's appointment as the UK's ambassador to the US was "catastrophically bad judgement", concluding that Sir Keir Starmer "should go".
UK unemployment rate falls
The UK umemployment rate fell to 4.9 per cent in the three months to February - this is down from 5.2 per cent in the three months to January, according to new data from the Office of National Statistics (ONS).
Also, the ONS have said average regular earnings growth has fallen to 3.6 per cent in the three months to February and was 0.4 per cent greater after taking Consumer Prices Index inflation into account.
In response to the figures, Pat McFadden, Secretary of State for Work and Pensions said: "These figures show that there was an improvement in the labour market at the beginning of the year with unemployment falling below 5 per cent, and 332,000 more people in work than a year ago.
“But we cannot escape the effects of the war in the Middle East which are likely to feed through to prices and employment in the coming months.
"We will do everything we can to support the country through this period, including by slashing energy bills by up to 25 per cent for 10,000 manufacturers.
Labour on brink of becoming FIFTH most popular party in Britain as brutal new polling emerges
Labour is now on the brink of being the fifth most popular party in Britain, a brutal new poll has revealed.
Fresh data from YouGov has found that just 16 per cent of Britons would vote for Sir Keir Starmer's party - now behind Reform UK, the Conservatives and even now the Greens.
Nigel Farage's party secured a three percentage point boost over last week's data, with 27 per cent of Britons opting for Reform.
It is the only party to receive more than a 17 per cent prospective vote share.
After falling by two points, Kemi Badenoch's Tories sit 10 adrift of Reform on 17 per cent, while the Greens fell by one to the same level.
The Lib Dems, after picking up one percentage point, sit in fifth place on 14 per cent.
Keir Starmer faces emergency debate on Lord Mandelson appointment in just hours
Sir Keir Starmer will face an emergency debate on Lord Mandelson's appointment today
| PASir Keir Starmer will face an emergency debate on Lord Mandelson's appointment as the UK's Ambassador to the US following an application by Tory leader Kemi Badenoch.
The Prime Minister was today hit with a barrage of criticism from MPs across the House of Commons after it emerged that Lord Mandelson did not pass security vetting.
Mrs Badenoch, who said "serious questions [remain] about what he knew and when" and warned the Prime Minister had "serious inconsistencies" in his position, had her pitch accepted by Commons Speaker Sir Lindsay Hoyle.
The debate is expected to start at around 12.30pm on Tuesday, shortly after Sir Olly Robbins provides evidence to MPs...
CONFIRMED: Exact time for Olly Robbins committee appearance
The Foreign Affairs Select Committee has confirmed Sir Olly Robbins will be appearing before MPs at 9am today.
You'll be able to follow the latest from his grilling live HERE - and by following along with our live blog here.
Meanwhile in Washington... Donald Trump brutally rips into Keir Starmer for appointing Lord Mandelson
In what will likely be seen as an untimely intervention from DC this morning, Donald Trump has laid into Sir Keir Starmer over his appointment of Lord Mandelson as ambassador to the US.
Mr Trump said: "Prime Minister Keir Starmer of the United Kingdom acknowledged that he 'exercised wrong judgement' when he chose his ambassador to Washington.
"I agree, he was a really bad pick."
"Plenty of time to recover, however," the President prodded.
READ THE FULL STORY ON DONALD TRUMP'S LATEST ATTACK ON THE PM HERE