Former senior official Sir Philip Barton has told MPs there was no way for him to raise his concerns about Lord Mandelson's appointment as the UK's ambassador to the US.
Sir Philip said no-one in Downing Street consulted him before making the decision to appoint Lord Mandelson when he was the top civil servant in the Foreign Office in 2024.
Speaking to the Foreign Affairs Committee, Sir Philip said he thought appointing Lord Mandelson could be a "potentially difficult issue" because of the Labour peer's known links to the late convicted sex offender, Jeffrey Epstein.
But he said he was "presented with a decision" made by Prime Minister Sir Keir Starmer and "told to get on with it".
Sir Philip said he knew Epstein was "a controversial figure" in the US, adding: "I was worried that this could become a problem in future."
"That is a very candid account of probably what I was thinking at the time, but there was no space or avenue or mechanism for me to put that on the table," Sir Philip said.
Sir Philip was appearing in front of the committee to answer questions about the process to vet Lord Mandelson before he took up the high-profile diplomatic role in Washington DC.
Sir Philip was permanent under-secretary at the Foreign, Commonwealth & Development Affairs (FCDO) between September 2020 and January 2025, a period that covered the process to appoint Lord Mandelson.
Lord Mandelson was announced as the UK's ambassador to the US in December 2024, with the peer then undergoing in-depth vetting to obtain his required security clearance for the role.
The former Labour minister was then sacked as US ambassador in September last year, after new details emerged about the extent of his friendship with Epstein. Lord Mandelson has since said he regretted ever having known Epstein.
Sir Keir appointed Lord Mandelson to the post, and the decision has dogged him for months, leading to questions about his judgement.
Later Sir Keir will face a vote by MPs on whether there should be a parliamentary investigation over his claims about the vetting of Lord Mandelson.
In his evidence to MPs on Tuesday, Sir Philip said he was first told about the decision to appoint Lord Mandelson on 15 December, 2024 - days before the Labour peer was announced.
When he was informed about the decision, Sir Philip said he was told "a due diligence process had been carried out and as is known now the prime minister had been made aware of the risks and accepted those risks and decided to proceed".
Sir Philip was asked whether he agreed with his successor at the Foreign Office, Sir Olly Robbins, who last week accused Downing Street of taking a "dismissive attitude" to vetting during Lord Mandelson's appointment.
Sir Philip said he would not describe No 10's attitude towards vetting as "dismissive".
Instead, the former official said Downing Street had been "uninterested" in vetting, and there were time pressures to complete the process before Donald Trump was inaugurated as US president in January 2025.
He said he was "not aware of any pressure on the substance" of Lord Mandelson's vetting coming from Number 10 and denied media reports that the prime minister's former chief of staff, Morgan McSweeney, ever swore at him over the case.
But Sir Philip said there was "absolutely" time pressure to get the vetting done quickly given "the top of the government is saying the prime minister has decided he wants Mandelson" in post.
McSweeney also gave evidence to the same committee on Tuesday morning.
In an opening statement, McSweeney apologised to the victims of Epstein for the distress caused by the controversy surrounding Lord Mandelson's appointment.
He said giving Lord Mandelson the job was "a serious error of judgement".
"The prime minister relied on my advice, and I got it wrong," McSweeney said.
He said he advised appointing Lord Mandelson based on his judgement that the Labour peer's "experience, relationships and political skills could serve the national interest in Washington at an important moment".
McSweeney said he had assumed, wrongly, that Lord Mandelson was "telling me the truth" about his relationship with Epstein.
But McSweeney said when the extent of Lord Mandelson's relationship with Epstein was revealed in a Bloomberg report last year, it was a "knife through my soul".
The committee hearing came ahead of a debate about the prime minister's comments on the vetting process in the House of Commons.
Opposition MPs have accused the prime minister of misleading Parliament over his assurances that "due process" was followed during Lord Mandelson's appointment and his assertion that "no pressure whatsoever" was applied to officials at the Foreign Office.
This afternoon, MPs will vote on whether the Privileges Committee should hold an inquiry into what the prime minister said about the vetting process.
The prime minister has denied the accusations and branded the move a "stunt" by the Conservatives.
The BBC understands Labour MPs are likely to be whipped to vote down the Conservative motion to refer him to the Privileges Committee rather than being given a free vote.
