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.

Collard was the official who briefed the then-Foreign Office boss Sir Olly Robbins about UK Security and 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 Robbins 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.

Defending his actions at Prime Minister's Questions, Sir Keir said that if the Foreign Office had told him about the vetting concerns "Mandelson would not have been committed to post".