Rachel Reeves held an undeclared meeting with Lord Mandelson, the latest release of the bombshell files has revealed.
The Chancellor solicited advice from the former US ambassador while he was the chair of a lobbying company which was trying to influence the Labour Government.
On November 18, 2024, then-Treasury chief of staff Baroness Martin said Ms Reeves was "keen to have a chat about trade" with Lord Mandelson, seen in newly-disclosed emails, unveiled as part of the Government's response to the Humble Address.
She asked if the Labour peer could "pop in" to advise on the issue before a meeting held the following day.
At the time, Lord Mandelson was chairman of Global Counsel, a lobbying firm he founded and which worked for the likes of JP Morgan, Shell, and Palantir.
No record of the meeting was publicly disclosed, despite the ministerial code requiring Government departments to publish details of ministier's meetings with external organisations and individuals.
The code reads: "Where a minister meets an external organisation or individual in an official capacity, details of these meetings must be declared. This applies to meetings with lobbyists."
A Treasury source said the meeting was not declared because it was not held in the Chancellor's "ministerial capacity" and suggested that because Lord Mandelson was a peer, he did not need to follow the declaration rules.
Rachel Reeves solicited advice from Lord Mandelson in an undisclosed meeting while he was chairman of lobbying firm Global Counsel
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The Conservatives have now accused the Treasury of "breaking its own transparency rules to allow ministers to receive advice from Mandelson while he was actively seeking to advance the interests of clients through Global Counsel".
But the Treasury said their claims were "nonsense", adding: "Treasury ministers comply with all relevant guidance."
The emails also revealed Lord Mandelson arranged to meet with Lord Livermore, the Financial Secretary to the Treasury.
In the exchange, the peer suggested bringing an unnamed Global Counsel employee to the meeting because he had "ideas".
MORE MANDELSON FILES REVELATIONS:
Lord Mandelson arranged two undisclosed meetings with Treasury officials, as revealed in newly disclosed messages and emails
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Lord Livermore would have informed the Treasury's work on growth, the digital economy and Net Zero - likely topic of interests among Global Counsel's clients, which included OpenAI and Sizewell C.
Treasury sources have said the meeting did not need to be declared as it was not held in Lord Livermore's ministerial capacity.
The two men had known for each other for more than 20 years, and both has served as Labour's General Election campaign directors.
In the WhatsApp messages, Lord Mandelson asked: "Are you around next week?"
Spencer Livermore (right) met with Lord Mandelson outside of the Treasury, joined by a Global Counsel employee
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Lord Livermore replied six minutes later asking if "Wednesday or Thursday would work", with Lord Mandelson suggesting to meet outside of the Treasury to get a "Marylebone bite".
Another meeting between a Global Counsel employee and then-Trade Minister Douglas Alexander was arranged by Lord Mandelson, but this was declared on the transparency records of the Department for Business and Trade.
Alex Burghart, the Shadow Chancellor of the Duchy of Lancaster, said: "The Treasury appears to be breaking its own transparency rules to allow ministers to receive advice from Mandelson while he was actively seeking to advance the interests of clients through Global Counsel.
"The public deserves to know who was meeting whom, what was discussed and why those meetings were not declared.
"Labour ministers need to come clean about the extent of their dealings with Mandelson.
"The Conservatives are the only reason we have any sort of transparency, and we will continue to fight until the Government holds themselves to account."