Keir Starmer will today be told to finally take responsibility for the Mandelson scandal and stop blaming everyone else.

The Prime Minister will begin a make-or-break week for his faltering premiership by facing angry MPs over his involvement in the 'tawdry and shaming' affair.

They are demanding to know how he could possibly have made Peter Mandelson this country's US ambassador without knowing he had failed security vetting.

He has already been hit by calls to quit from Labour backbenchers and peers as well as opposition party leaders over his 'catastrophic' misjudgments.

But Sir Keir signalled he would not be taking the blame and last night said he would make it 'crystal clear' to MPs that he was kept in the dark.

He will say that senior civil servant Sir Olly Robbins should have told him about the red flags raised over the controversial appointment – both at the time and when he later wrongly assured the Commons that it had been done by the book. Sir Olly was sacked last week and has been asked to appear in front of the foreign affairs committee tomorrow.

Ministers insisted yesterday that Sir Keir would not have appointed Mandelson if he had known about the concerns raised.

But many in Westminster believe it is entirely the PM's fault for giving Mandelson the prestigious post in Washington DC despite his chequered past – and for failing to ask if he had passed checks carried out by his Government's own security agency. 

Last night he was told by Tory leader Kemi Badenoch: 'Instead of another round of blaming everyone else and demeaning the office of Prime Minister, I trust we can look forward to a frank statement in which you will take due responsibility.'

In a damning letter to No10, she accused Sir Keir of breaking the Ministerial Code by failing to be open with MPs and for not revealing details of the vetting failure as soon as he was told about it last Tuesday.

Two of the country's most senior civil servants had been aware of the fiasco for weeks and critics say the PM could have addressed MPs, armed with the full facts, much sooner. 'No more evasion. No more digressions. Parliament and the public will want to hear you give us the truth, the whole truth and nothing but the truth,' Mrs Badenoch wrote.

She said the wider Mandelson affair had been 'tawdry and shaming', accusing the PM of damaging Britain's relationship with the United States as well as undermining national security.

Liberal Democrat leader Sir Ed Davey accused Sir Keir of 'catastrophic misjudgment' and said the failure to check on the vetting process had been 'totally incompetent'. The Lib Dems are calling for Sir Keir to face an inquisition from MPs on the privileges committee as well as an investigation by the Government's ethics watchdog for alleged breaches of the Ministerial Code.

Labour peer Lord Glasman became the most prominent figure in Sir Keir's own party to call for him to resign yesterday, saying: 'He cannot conceivably continue as a credible Prime Minister any longer. And that's all because he cannot say, "I made a mistake, I'm sorry".'

When asked last night if he would apologise to MPs, Sir Keir told the Mirror: '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.'

Science and Technology Secretary Liz Kendall admitted 'this whole torrid episode has been an extremely testing time', but insisted: 'Keir Starmer is my favourite to be the Prime Minister to take us into the next election.'

Sir Olly is expected to give evidence to the foreign affairs committee tomorrow, but as of last night, he had still not replied to its invitation. He is thought to be taking legal advice after he was sacked by the Prime Minister for failing to admit he had cleared Mandelson to take up the ambassador's job against the advice from UK Security Vetting officials.

Sir Keir told broadcasters he was 'absolutely furious' at the 'staggering' omission.

But allies of Sir Olly have insisted he was not allowed to tell anyone about the results of the vetting exercise and so there was no basis for him to be dismissed.

One former official told the Financial Times: 'Starmer has been casting around for someone to blame, other than himself.

'It wasn't just the firing, it was the pile-on afterwards. It's another person thrown under the bus.'

The Government published a statement last night attempting to rebut Sir Olly's defence, claiming: '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.

'This allows ministers to make informed decisions, including on appointments or when accounting for government business in Parliament.'

10 Questions the PM has to answer

Keir Starmer addresses the Commons on the Peter Mandelson vetting scandal today. MARTIN BECKFORD says these are the questions MPs will want answered:

1. Did Sir Keir mislead Parliament?

The Prime Minister told MPs on several occasions that 'full due process' was followed, including 'security vetting', before Mandelson took up his role. His right-hand man Darren Jones insisted last week this was technically correct because the Foreign Office had a little-known power to overrule vetting concerns. But Sir Keir will face damaging accusations that he did not tell the truth, regardless or whether he knew the full facts at the time.

2. Why didn't the PM correct the record immediately?

The Ministerial Code requires members of the Government to correct 'any inadvertent error at the earliest opportunity' if they did not give accurate information to MPs. Sir Keir was told about the vetting scandal on Tuesday night so will be asked why he didn't correct the record on Wednesday, before Prime Minister's Questions.

3. Was Sir Keir really the last to know?

Cabinet Office Permanent Secretary Cat Little was handed a document revealing the vetting failure in late March and discussed it with the country's top civil servant, Dame Antonia Romeo. The PM was told weeks later. Sir Keir will be asked if he is happy that they kept him in the dark – and how many others in No10 knew before him.

4. Should he have been told what had happened?

Foreign Office mandarin Sir Olly Robbins, who made the fateful decision to give Mandelson the job, believes he was not allowed to tell anyone about the concerns that emerged during the vetting process. But many who have worked in Whitehall say they had been informed when the alarm had previously been raised over other candidates.

5. Why did officials say Mandelson should not get security clearance?

Although the New Labour grandee has been linked to Russia and China as well as paedophile Jeffrey Epstein during his long and colourful career, it is not yet known what exactly led UK Security Vetting officials to raise the red flag over his appointment to Washington.

6. Has the PM seen the verdict himself yet?

Last month No10 officials finally got hold of the document revealing that 'the recommendation from the vetting officer had been that developed vetting should not be granted to Peter Mandelson'. But Sir Keir may have been told that the reasons behind the advice have to remain secret to protect Mandelson's privacy.

7. Did Mandelson read top-secret reports?

As well as developed vetting, Mandelson needed another type of clearance known as STRAP so he could be shown intelligence material. Did he gain this despite failing the checks, or was he allowed to see the top-secret files anyway?

8. How much digging did Downing Street do?

Journalists went to No10 as early as September with claims that Mandelson had failed his vetting but were assured that the normal process had been followed. Downing Street has insisted that officials had repeatedly asked questions of the Foreign Office about the process, but it is not known exactly what they asked or what responses they received.

9. Why didn't Foreign Secretary know?

David Lammy, in charge of the Foreign Office at the time, revealed yesterday he heard about the debacle only last Thursday. But Tory predecessor Sir James Cleverly said it should have been his job to find out.

10. Was the 'fall guy' only obeying orders?

Sir Olly, who was sacked by Sir Keir last week, told MPs in November: 'It was clear that the Prime Minister wanted to make this appointment himself.' Did the PM or his then chief of staff Morgan McSweeney make it known that their candidate had to be put in place regardless of any concerns raised?