Peter Mandelson's ties to China, Russia and Israel were among some of the concerns raised by the UK's vetting agency.
Britain's top security services confirmed the peer's appointment as the UK Ambassador to the US should have not been approved due to his association with a number of officials across the globe.
The New Labour titan was revealed to have been acquainted with China's finance minister Lan Fo'an, as well as the sanction-laden Russian oligarch Oleg Deripaska.
Lord Mandelson is also said to have been associated with Israeli military intelligence general, Tamir Hayman.
A spokesman for Mr Hayman, however, told The Guardian that he "has no personal connection or familiarity whatsoever".
He added that any contact between the pair was regarding work for Mr Hayman's think tank.
The former Cabinet minister continued to retain his links with Russia's top business officials following Vladimir Putin's invasion of Crimea back in 2014.
Nevertheless, the disgraced peer was catapulted into the top role by the Foreign Office despite failing to acquire the necessary security clearance.
The next batch of documents is expected to be released as soon as next week
|GETTY
His international connections were exposed after the Prime Minister's Chief Secretary announced that the second batch of files detailing the peer's vetting process would be delayed.
Last week, ministers were hauled before their Commons colleauges to accusations it had "withheld and redacted key information" regarding the appointment of Peter Mandelson as UK Ambassador to the US.
Sir Lindsay Hoyle allowed the urgent question on the matter, which was tabled by a member of the intelligence and security committee (ISC).
The group of MPs has been tasked with overseeing the publication of the materials following the Tories' Humble Address, the mechanism to force Labour's hand into releasing the papers, earlier this year.
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Darren Jones announced the delay in the Commons last week
| PARLIAMENTMembers of the committee, including Sir Jeremy Wright and Dame Emily Thornberry, have hit out at Sir Keir Starmer's team for "withholding" information and redacting data "far too broadly".
The announcement of the delay has triggered claims of a "cover up", with Tory frontbencher Neil O'Brien taking aim at the redactions as well.
While some data deletions were previously agreed on between No10 and the parliamentary committee, Downing Street's headache from the inquiry into the disgraced peer has persisted.
The alarm over his international associations were sounded back in 2025 when Donald Trump flirted with rejecting the Labour lord as Britain's ambassador in the US due to his reported links to China.
His connections abroad have been exposed at the same time it has been revealed that the peer had handed out advice to a gaggle of ministers during his time in the diplomatic role.
Explicit material detailing the "mostly unsolicited" advice is expected to be unveiled when the next batch of documents is released, which is expected to be next week.
According to the Telegraph, sources have ceded that Lord Mandelson was not normally approached by Sir Keir's top team - but shared his two cents' nonetheless.
The upcoming release of the documents is likely to pile pressure onto the Prime Minister to give up his premiership amid furious leadership speculation.
A government spokesman said: “We are committed to complying with the Humble Address in full.”
GB News has approached Lord Mandelson's lawyers for comment.