Nick Eardley,Political correspondentand
Paul Seddon,Political reporter
The prime minister will appear before MPs on Monday to explain what went wrong, after it emerged that Lord Mandelson did not pass inital security vetting checks ahead of taking up the role of ambassador to the United States.
But there are some key questions that remain unanswered.
Did the prime minister and the government mislead MPs – and the public?
The prime minster has said he only found out this week that Lord Mandelson had failed developed vetting (DV) – an intrusive process designed to ensure that people with access to secret information, like ambassadors, do not pose a potential security risk.
A day before Lord Mandelson was sacked last September, the prime minister told MPs that “full due process” had been followed during his appointment.
But the day after the peer was dismissed, The Independent raised questions about that process, reporting that Lord Mandelson may not have passed security checks.
At a press conference in February this year, Sir Keir said the vetting process “gave him clearance for the role”.
On Friday, he insisted he “was not told” the peer had failed vetting, something he described as “staggering” and “completely unacceptable”.
But opposition parties have cast doubt on his version of events. Conservative leader Kemi Badenoch has branded his explanation “completely preposterous”.
Why did the PM therefore say at the time that ‘full due process’ had been followed?
And added to that question: why in February this year did he go further, saying there was “security vetting carried out independently by the security services… gave him clearance for the role”?
Which leads to the next question…
Why weren’t more questions asked by the PM and his team?
Given journalists were asking about Lord Mandelson failing vetting, should No 10 have been more curious?
No 10 said there had been repeated requests for “assurances” from the Foreign Office about the “facts of this case”, including vetting.
Why did Mandelson fail vetting?
Developed vetting is said to be a highly personal process. It’s unlikely we will see full details of the questions asked and answers given.
But will the public be told what the concerns were?
Security vetting is carried out by United Kingdom Security Vetting (UKSV), a specialist agency within the Cabinet Office, and is a requirement for almost all Foreign Office officials.
The government has said that officials at UKSV recommended against granting Lord Mandelson security clearance – but has not divulged why.
According to government guidance, security clearances can come with strings attached, such as restrictions to manage conflicts of interest.
The BBC has been told that UKSV presented the Foreign Office with a list of potential risks, as well as a recommendation summing up those risks.
That recommendation can fall into one of three categories, described by sources as “yes”, “yes with caveats” and “no”.
Sources say the recommendation given to the Foreign Office by the vetting services was a “no”.
Why were those concerns overruled?
And alongside that – why did officials in the Foreign Office make the decision to go ahead with the appointment?
Lord Mandelson’s job had already been announced – would it have been too embarrassing to cancel it? Or were mitigations found to address the concerns? At the moment, we simply don’t know.
Who approved Mandelson’s clearance?
The government has said the decision to grant his security clearance was made by “officials” within the Foreign Office – without specifying who exactly made the call.
Giving evidence to MPs last November, Sir Olly said that most cases would not require a decision to be made at the most senior level, but that cases can be “escalated appropriately”.
Asked directly whether Lord Mandelson’s case was escalated, Sir Olly replied: “I certainly cannot comment on that, I’m afraid”.
The security vetting took place alongside a separate process, run by the Foreign Office, to assess Lord Mandelson’s financial conflicts of interests.
Sir Olly told MPs he had put “some measures” in place to mitigate potential conflicts arising from the peer’s links to Global Counsel, the lobbying company he co-founded in 2010.
Who was told about the vetting recommendation?
The government has said no ministers, including Sir Keir, knew that Lord Mandelson had failed the vetting process at the time – and were not made aware until earlier this week.
A spokesperson for Sir Keir said on Friday that no one in No 10 Downing Street, “officials or otherwise”, knew Lord Mandelson had failed his security vetting, despite repeated requests for “assurances.”
In a letter to MPs last September, Sir Olly said the security vetting process for Lord Mandelson was carried out independently of ministers, who were not informed of any “any findings other than the final outcome”.
The BBC understands Lord Mandelson had no knowledge at the time, or since, of the judgements reached during his vetting process.
What will Labour MPs do?
Plenty of Labour MPs are angry with the prime minister – and have been for some time.
They didn’t act earlier this year when there were calls for the prime minister to quit. Will that change this time?
At the moment, many are busy campaigning ahead of May’s elections in Scotland, Wales and for some councils in England.
Some are speaking up, but I don’t yet detect a groundswell of pressure.
The PM’s answers on Monday, when he makes a statement in the House of Commons, will be studied closely.