A key figure in the row over Lord Mandelson's vetting will not appear before a parliamentary committee of MPs to give evidence.
The Foreign Affairs Committee had asked the Foreign Office if Ian Collard, a civil servant who ran the security team within the department, could attend next week.
But the committee's chair Dame Emily Thornberry said the department made the "decision to decline" the request and Collard will only be giving evidence in writing.
She added, on X: "To be clear, I am satisfied by the reasons behind Ian Collard not giving oral evidence.
"If we have further questions, we will consider at that point whether we need to ask him to give evidence orally, or whether a further written statement is sufficient".
Collard was the official who briefed the then-Foreign Office boss Sir Olly Robbins about UK Security Vetting's (UKSV) recommendation not to give clearance to Mandelson.
Sir Olly was sacked last week after it emerged that he had granted clearance against the recommendation and had not informed No 10.
The government says UKSV gave an explicit recommendation to the Foreign Office not to approve vetting for Lord Mandelson ahead of his confirmation as ambassador to the US.
But speaking to MPs on Parliament's Foreign Affairs Committee this week, Sir Olly said he had never seen that explicit recommendation and only received a verbal briefing which described UKSV's view as "borderline" and "leaning towards recommending that clearance be denied".
The government is investigating whether Sir Olly was given the correct information before he approved security clearance for the peer.
Sir Adrian Fulford, a retired judge, is conducting a review into the process and it is understood he will look at whether the briefing given by Collard correctly summed up the vetting team's view.
That information could be crucial to determine whether Sir Keir Starmer was right to sack Sir Olly last week.
In a letter to the interim Foreign Office boss, Dame Emily set out some questions for Collard to answer in writing, including: "How often did his team make a different recommendation on vetting to that contained in the UKSV report?"
The Foreign Office has not commented on Collard not appearing in front of the committee.
Sir Keir's former chief of staff Morgan McSweeney will give evidence to the committee on Tuesday.
McSweeney, who advised the prime minister to appoint Lord Mandelson as the UK's ambassador to the US, is likely to face questions about whether he put pressure on civil servants to speed up the vetting.
It comes after Sir Olly accused No 10 of a "dismissive" attitude towards the process - a claim Downing Street has denied.
The row over Lord Mandelson's vetting has reignited questions among Labour MPs about Sir Keir's judgement and leadership.
In an interview with the Sunday Times, Sir Keir said Sir Olly faced only the "everyday pressure of government" during the process.
"There are different types of pressure," he said. "There's pressure, 'Can we get this done quickly,' which is not an unusual pressure."
And asked whether he regretted sacking Sir Olly so quickly, Sir Keir said: "I do not accept the argument that that is something which should not be told to the prime minister."
