The chief Foreign Office official ousted over the decision to grant Peter Mandelson a security clearance despite vetting concerns is expected to face questions from MPs on Tuesday.
Sir Olly Robbins will be grilled by the Foreign Affairs Committee over why the ex-US ambassador received the clearance, and whether No 10 was told about red flags.
It adds to the potential jeopardy Sir Keir Starmer faces next week, with the planned appearance coming a day after he is due to appear in the Commons to answer questions about the row.
Lord Simon McDonald, who was permanent secretary between 2015 and 2020, told BBC Radio 4's Today programme that "No 10 wanted a scalp and wanted it quickly".
He continued: "I cannot see that there was any process, any fairness, any giving [Sir Olly] the chance to set out his case, and that feels, to me, wrong."
The BBC understands that those close to Lord Mandelson believe the sacking of Sir Olly as permanent secretary is "egregious".
Lord McDonald said details from the "confidential" vetting process would "never be shared with No 10 or the prime minister".
An outright failure of the vetting process "would have to be conveyed to the political level", Lord McDonald added - and the fact it appears not to have been indicates "the picture was more complicated than Number 10 wished to present".
But multiple Whitehall sources have told BBC News security officials did outrightly recommend against Lord Mandelson's security clearance.
On Friday the government published the UK Security Vetting's decision template, the type of document they would have been presented with in Lord Mandelson's case.
It shows security officials typically indicate a low, moderate or high level of concern followed by an explicit recommendation that clearance be approved, approved with risk management, or denied. Both sets of options are reflected by green, yellow, or red boxes.
The BBC has been told that in Lord Mandelson's vetting, officials ticked the red boxes in both instances, indicating the highest level of concern, and explicitly recommending his security clearance be denied.
Sir Keir has come under intense scrutiny from opposition MPs, who have sought to cast doubt on his insistence that he first learned Lord Mandelson had failed the vetting process earlier this week.
The prime minister had previously told the Commons that "due process" was followed over Lord Mandelson's appointment, prompting accusations from opposition MPs that he misled Parliament.
On Friday evening, Downing Street took the unusual step of releasing an official account of the meeting on 15 April during which the prime minister has said he was informed about Lord Mandelson's vetting for the first time.
He has labelled the decision not to inform him sooner as "staggering", adding: "That I wasn't told that he had failed security vetting when I was telling Parliament that due process had been followed is unforgivable."
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What we know about Mandelson's vetting and security clearance
It has also emerged that Cabinet Secretary Antonia Romeo found out about Mandelson's vetting failure in late March, but had to investigate the circumstances and legal implications before informing the prime minister.
Mandelson was announced as the government's choice to be US ambassador in December 2024, with developed vetting beginning the same day.
He formally took up the post in February 2025, before being removed last September when further details about his past ties with convicted sex offender Jeffrey Epstein came to light.
Sir Olly had been the most senior civil servant at the Foreign Office for two weeks when it was announced Mandelson had been picked for the Washington role.
Dame Emily Thornberry wrote to Sir Olly on Friday requesting that he appears before the Foreign Affairs Committee she chairs to answer questions.
She said developments this week had "called into question" evidence Sir Olly previously gave to the committee.
Foreign Secretary Yvette Cooper has written to Dame Emily, saying she is "extremely concerned" that information about the security vetting "was not previously provided to ministers - which has in turn affected information given to Parliament".
She said she had asked the Foreign Office to conduct a full review of the information sent to the committee on the process undertaken as part of Lord Mandelson's appointment.
"It is of paramount importance that the information... is fully accurate," she wrote.
BBC News understands Sir Olly has not formally accepted the committee's invitation, but friends of his said he was preparing to appear on Tuesday.
They also cast doubt on whether Sir Olly ever actually saw the recommendation to the Foreign Office not to approve Lord Mandelson's vetting.
Allies said it was unlikely he was given the full report and may only have seen more specific risks thrown up during the process.
BBC News understands the vetting report returned a "no" verdict on whether a security clearance granting access to sensitive government material should be given to Mandelson.
Vetting of government employees and appointees is carried out by a government agency overseen by the Cabinet Office, which reports back to hiring departments with recommendations.
Following Sir Olly's departure, Conservative leader Kemi Badenoch said the prime minister had "no one left to sack" and described his position as untenable.
Liberal Democrat leader Sir Ed Davey called for an investigation by the Privileges Committee into whether the prime minister intentionally misled Parliament.
The SNP, Green Party and Reform UK have also called for Sir Keir to resign.
DW Russian 
