Keir Starmer is facing a reckoning on the Peter Mandelson scandal today as a Cabinet minister admitted it is not certain he will lead Labour into the next election.

The PM will run the gauntlet of MP fury in the Commons this afternoon as he desperately tries to shift the blame on to another 'fall guy'.

Sir Keir has ousted Foreign Office chief Olly Robbins, insisting it was 'unforgivable' the civil service did not tell him vetting officials advised against making Mandelson US ambassador.

But Sir Olly is believed to be consulting lawyers with allies adamant the mandarin did nothing wrong by going ahead with the New Labour architect's security clearance.  

In a significant escalation last night, the Government released what appeared to be internal legal advice suggesting there was no barrier to Sir Olly flagging the vetting conclusions. 

The latest turmoil has renewed speculation about Sir Keir's future, with the PM having barely survived a coup attempt in February. 

Scottish Secretary Douglas Alexander risked inflaming the situation this morning by acknowledging there are doubts about the premier's future.  

Asked on Sky News whether Sir Keir will lead Labour into the next election, due in 2029, Mr Alexander said: 'I expect so, yes… I think he will.'

He added: 'There are no certainties but but of course I think he will lead and I think he should because, frankly, on the biggest call in this parliament he's exercised the right judgment, which is to keep us out of someone else's war.'

Mr Alexander said 'rightfully and reasonably' there were 'important questions that need to be answered today'.

'Keir Starmer is going to set out all the facts, the right place for those questions to be answered are at the despatch box of the House of Commons,' he said.

Labour's leader in Scotland, Anas Sarwar, has already declared he has no confidence in Sir Keir and would rather he was replaced. 

Lord Mandelson was sacked last year, just nine months into the Washington DC posting, after further details of his ties to paedophile financier Jeffrey Epstein emerged. 

It is not clear why vetting officials advised against his appointment, and allies insisted he was not aware of the recommendation until last week. 

Despite the advice, he is believed to have been granted the highest 'Strap 3' level of security clearance. 

Whitehall veteran Sir Olly is expected to give his own account to MPs at the Foreign Affairs Committee tomorrow. 

A statement issued by No10 last 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'.

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 PM 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.'

But Tory leader Kemi Badenoch has said Sir Keir is 'either lying or he's incompetent'.

Mrs Badenoch said: 'This has been a tawdry and shaming affair for you and your party, and for this country.

'Not only have you damaged our relationship with the United States and insulted the victims of the paedophile Jeffrey Epstein, but you have also undermined our national security by giving the highest diplomatic post to an individual that the security services found to be of 'high concern'.'