Sir Keir Starmer has admitted he was “wrong” in his judgement to appoint Peter Mandelson as US ambassador in a fresh Commons showdown over the scandal.
The Prime Minister is laying out further details of the appointment process after it was revealed Lord Mandelson, who had a close relationship with paedophile financier Jeffrey Epstein, had failed security vetting.
Sir Keir said: "I want to be very clear with the House that, while this statement will focus on the process surrounding Peter Mandelson's vetting and appointment, at the heart of this, there is also a judgement I made that was wrong.
"I should not have appointed Peter Mandelson. I take responsibility for that decision.
“I apologise, again to the victims of the paedophile, Jeffrey Esptein, who were clearly failed by my decision."
The scandal has fueled further calls for Sir Keir to resign, both from opposition parties but also from critics within the Labour movement who already fear an electoral bloodbath in May's local elections.
Sir Keir has faced allegations he misled Parliament when he told MPs the proper process had been followed in appointing Lord Mandelson, insisting he had been kept in the dark about the peer being red-flagged by security experts.
The Foreign Office's top official Sir Olly Robbins was last week effectively fired by Sir Keir after it emerged Lord Mandelson had been given developed vetting (DV) status despite failing checks carried out by the agency responsible for assessing security clearances.
The Prime Minister's defence will be to blame officials for not telling him, or the then foreign secretary David Lammy, that UK Security Vetting (UKSV) had not cleared the peer.
Lord Mandelson was sacked last year, just nine months into the Washington DC posting, after further details of his association with Epstein emerged, a relationship which was widely known about before his appointment.
Whitehall veteran Sir Olly is expected to give his own account of the events to MPs on Tuesday at the Foreign Affairs Committee.
Scottish Secretary Douglas Alexander said on Monday that he expected Sir Keir to survive to lead Labour into the next general election "but there are no certainties" in politics.
He also defended the decision to force out Sir Olly, telling Times Radio: “The reality is there is a growing body of evidence, not least including allies and friends of Olly Robbins, who accept that he did not share that information with the Prime Minister.
“Now there is disagreement as to whether and why he didn't share that information, but the central charge against the Prime Minister, that somehow he knowingly misled the public or parliament, is now reliant on what would need to be a growing conspiracy of every minister involved in this process, not just the Prime Minister, a growing number of senior civil servants, all of whom accept that Olly Robbins did not share that information with the Prime Minister.”
A statement issued by No 10 on Sunday night said that although civil servants rather than ministers make decisions on vetting and clearance, there was nothing in the law to prevent ministers being told.
"There is nothing in the guidance which prevented information being shared in this scenario, in a proportionate and necessary way and subject to the appropriate procedural steps," the statement on the Constitutional Reform and Governance Act said.
While there are "legal obligations" under data protection rules, "no law prevents civil servants - while continuing to protect such sensitive personal information - from sensibly flagging UK Security Vetting recommendations or high level risks and mitigations".
UKSV's privacy notice sets out there are "limited circumstances in which relevant vetting information can be shared" if "a security risk has been identified".
Mr Alexander also suggested that Foreign Office officials would have made a decision to allow Lord Mandelson access to top secret "strap three" material, as reported by The Times.
"When strap clearance is given to someone, the process goes through the department that is sponsoring the application," he added.
Sir Keir told the Mirror he would make it "crystal clear" to MPs that he had been kept in the dark and it was "unforgivable" that the Foreign Office failed to tell him after he had offered public assurances that proper process had been followed.
The Prime Minister said: "The fact that I wasn't told that Peter Mandelson had failed his security vetting when he was appointed is astonishing. The fact that I wasn't told when I said to Parliament that due process had been followed is unforgivable, and that's why I intend to set out in Parliament on Monday the facts behind that, so there's full transparency in relation to it.
"But am I furious that I wasn't told? Yes, I am. Am I furious that other ministers weren't told? Yes, I am. I should have been told, and I wasn't told."
The Prime Minister will face accusations he misled Parliament - potentially a resignation matter - Tory leader Kemi Badenoch has claimed Sir Keir is "either lying or he's incompetent".
Asked if he would apologise to MPs on Monday, the Prime Minister said: "I'm going to set out in terms what happened.
"But I shall be making it absolutely crystal clear, as I have done a number of times, and I don't think anybody is disputing this, that I was not told that Peter Mandelson had failed his security vetting, and I should have been told."
He will say the information should have been provided to both him and MPs a long time ago.
Instead, the Prime Minister was only informed about the vetting issue on Tuesday evening after the information was uncovered as part of the process of gathering files related to Lord Mandelson's appointment to comply with an order by MPs to release all relevant documents.
Allies of the Prime Minister insisted that Monday was the first opportunity he has had to set out the full facts to Parliament, despite appearing in the Commons on Wednesday for his regular question time session.
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