Messages between senior minister Darren Jones and Peter Mandelson have come to light, with the pair criticising Jones's cabinet colleagues.
The chats were expected to appear in the batch of Mandelson files, published earlier this week following MPs' vote to disclose documents relating to the disgraced peer's appointment as UK ambassador to the US.
Lord Mandelson was sacked from that role when the depth of his relationship with paedophile financier Jeffrey Epstein became apparent.
Conversations between Jones and Lord Mandelson did not appear in the 1,500 pages of documents published on Monday, but some messages have been published in the Spectator.
They include messages from Jones praising Lord Mandelson after he was sacked and discussing avenues for promotion within government.
On the day Sir Keir Starmer dismissed Lord Mandelson from his ambassadorial role, Jones wrote: "You've been doing such a great job, and you worked wonders with Trump. I'm so sorry about today."
Messages show Lord Mandelson criticised the government's plans for growth, telling Jones the plans were in the hands of Chancellor Rachel Reeves, then-deputy prime minister Angela Rayner, and then-business secretary Jonathan Reynolds.
Jones, who at that point was Reeves' number two, replied: "It doesn't fill you with confidence."
In another exchange about Reynolds' special advisers and industrial policy advisers, Jones said: "I lost faith in his spads when, on a call about Port Talbot, they repeatedly took a different position to us in HMT [the Treasury] 'because that's what the unions want'."
Jones also sought Lord Mandelson's advice on promotion during last year's Cabinet reshuffle, sparked by Rayner's resignation as deputy prime minister and housing secretary over a tax row about unpaid stamp duty.
Talking about his ambitions, Jones said his first preference was a role in the Department for Business and Trade, then headed by Reynolds but now led by Peter Kyle.
That was followed by the Department of Science, Innovation and Technology, then led by Kyle and now by Liz Kendall, or the Department for Energy Security and Net Zero, led by Ed Miliband both before and after the reshuffle.
Jones added: "I also like MoD but think that's unlikely."
Jones told Lord Mandelson that was because he thought defence secretary John Healey was doing a good job but suggested Reynolds may not be, saying: "DBT my preference – everyone fond of Jonny but perception that DBT not firing on full cylinders…"
Reynolds was reshuffled to chief whip and Jones ended up with the plum role of Chief Secretary to the Prime Minister and Minister for Intergovernmental Relations.
Responding to an earlier debate on the Mandelson files in the House of Commons on Wednesday, Jones told MPs he had turned on the disappearing messages feature in WhatsApp, which is allowed under current ministerial guidance.
He said: "Moving to the documents that Members may have expected to see in the second tranche, as I said on Monday, some messages may not have been captured where people may have previously changed their phones without having backed up their messages or where they had disappearing messages turned on, and I noted to the House on Monday that that included myself.
"In my circumstance, to answer the questions from the shadow Chancellor of the Duchy of Lancaster [Alex Burghart], it is not that I took a unilateral decision about messages that I felt were in scope or not in scope of the Humble Address; it is merely that I have access to no messages to disclose."
In the debate, Jones also said he may have inadvertently given Lord Mandelson too much leeway and offered an apology, including to Epstein victim Lisa Phillips.
He said he had not consciously ignored negative stories about Lord Mandelson and denied having been given any warnings about the Labour veteran.
"Did I at best subconsciously treat Peter Mandelson differently because I believed him to have influence and power within the Labour party?" he said.
"I think the answer to that question is yes, I did. Have I benefited from that relationship in the time I have been an elected politician? I think in part the answer to that question is yes, I did.
"For that I would like to apologise to the House, to the victims, to Lisa, and commit to then doing something about it."
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