Sir Keir Starmer faces allegations of violating the ministerial code after reportedly failing to disclose a visit to technology company Palantir's Washington headquarters in February 2025.
The Prime Minister was accompanied by Lord Mandelson, who was serving as Britain's ambassador to the United States at the time.
Under ministerial code requirements, ministers must publicly record their engagements with external organisations.
However, no official documentation of the Palantir visit appeared in transparency records, and officials took no minutes.
Palantir was a client of Global Counsel, the lobbying firm established by Lord Mandelson in 2010, in which he maintained a shareholding until shortly before its recent collapse.
A breach of the ministerial code is generally considered grounds for resignation.
The visit took place on February 27 at Palantir's offices in the American capital, where Sir Keir received a brief presentation on the company's operations followed by a tour of the premises.
Britain's defence attaché to the United States attended alongside 11 military personnel and additional British officials.
Sir Keir Starmer's trip to Palantir was not included in transparency records
|GETTY
The existence of the engagement came to light through a LinkedIn post stating: "Living the Special Relationship: Palantir hosted UK Prime Minister Keir Starmer at our Washington DC office.
"Notably, while other visits from the same overseas trip appeared on Sir Keir's transparency returns, the Palantir engagement did not.
Government sources confirmed embassy officials in Washington arranged the visit, although they refused to say whether Lord Mandelson played any direct role in organising it.
Downing Street maintains the Palantir engagement did not constitute a formal meeting and therefore required no official declaration.
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Sir Keir Stamer has been accused of breaching ministerial code | GETTYCabinet Office minister Nick Thomas-Symonds told Parliament the visit was "not a formal meeting" when responding to Conservative questions about the missing disclosure.
Yet the Ministry of Defence offered a contradictory account. Defence minister Luke Pollard acknowledged it was indeed a meeting, stating: "The Office of the UK Defence Attaché holds no record of the meeting as no formal record of the meeting was produced."
No10 sources characterised the engagement as a "routine visit" where taking no minutes was "standard practice", noting the visit itself was publicly known.
Government guidance indicates whether overseas engagements qualify as meetings is determined on a "case-by-case basis".
Palantir has maintained a longstanding relationship with the Ministry of Defence stretching back more than a decade. Later in 2025, the software company secured a five-year contract worth £75million with the MoD.
Lord Mandelson's shareholding in Global Counsel meant he stood to benefit financially from any business the lobbying firm's clients obtained.
Upon taking up his ambassadorial post, Lord Mandelson retained a substantial stake in Global Counsel. He committed to gradually divesting his shares and placing them in a blind trust pending their sale.
A Palantir spokesman insisted no discussions about future contracts occurred during the technology demonstrations.
The company stated its latest MoD contract, renewing a 2022 agreement, was first discussed with defence officials before Lord Mandelson became ambassador and finalised more than three months after his dismissal.
The Conservatives have branded the failure to record the Palantir engagement an "extremely serious" breach of ministerial rules.
Shadow Chancellor of the Duchy of Lancaster, Alex Burghart, told The Sunday Telegraph: "Keir Starmer has broken the ministerial code. The public deserves to know who arranged this meeting, what was discussed, and what Global Counsel's client stood to gain.
"Mr Burghart is urging Scotland Yard to expand its existing investigation into Lord Mandelson to encompass potential misconduct connected to the Palantir visit."
He added: "A presentation by a defence contractor, attended with British military personnel, took place and was not declared. Calling it 'not a meeting' does not make it disappear."
The allegations emerge as Sir Keir struggles to steady his leadership following a turbulent week that saw Cabinet allies turn against him over his handling of the Mandelson affair.
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