Lord Mandelson leaked sensitive UK government tax plans to Jeffrey Epstein when he was business secretary, emails appear to show.
The confidential document, released by the US Department of Justice as part of millions of files relating to Epstein, was written by Nick Butler, a special adviser to Gordon Brown, the prime minister at the time.
Mandelson forwarded the memo to Epstein in 2009, captioning it: "Interesting note that's gone to the PM."
The revelation will add to the pressure on the former British ambassador to Washington, who resigned his membership from the Labour Party yesterday.
Lord Mandelson has been approached for comment.
On his relationship with Epstein he has previously said: "I was wrong to believe Epstein following his conviction and to continue my association with him afterwards. I apologise unequivocally for doing so to the women and girls who suffered."
Butler told Paste BN: "I don't have any comment beyond being shocked by the betrayal of trust involved."