The BBC said it had recovered the "original handwritten note" that the princess wrote following the Panorama interview of November 1995.
The broadcaster said it would hand over the note to an independent inquiry.
The probe will look at claims made by Diana's brother about how BBC reporter Martin Bashir secured the interview.
Bashir, 57, currently BBC News religion editor, is recovering from heart surgery and complications from Covid-19 and has been unable to comment on Charles Spencer's allegations.
Earl Spencer called for an independent inquiry earlier this month and said he would never have introduced Bashir to his sister if he had not seen the faked bank statements.
The faked statements wrongly purported to show that the earl's former head of security had been paid by a newspaper group and a mysterious offshore company.
The BBC has apologised for the faked bank statements, but it says the note from the princess says she did not see them and insisted they played "no part in her decision to take part in the interview".
The BBC's not saying how the letter came back into its possession - but it clearly hopes it will help its case when the independent investigation looks into the allegations made by Earl Spencer.
The letter is believed to say that Diana was not influenced by the forged bank statements Martin Bashir had made - and was happy with the way the interview was secured.
If the princess was unaware of or untroubled by the forgeries, or the alleged deceit, it will help the BBC's defence: it says the original investigation was into whether the princess had been misled into giving the interview.
A note from her saying she hadn't would clearly weigh heavily. But the note does not address Earl Spencer's central allegation.
He alleges that the forged documents were part of a series of lies he was told by Bashir, lies that were meant to win his trust and thus gain access to Diana.
What we know of the rediscovered note from Diana doesn't address the serious allegations of journalistic misconduct Earl Spencer has made.
Nor does it help resolve the question of how much the BBC knew back in 1996 when it said the forgeries played no part in securing the interview.
Nearly 23 million people tuned in to watch the Panorama interview, recorded almost 25 years ago, on 20 November 1995.
The interview made headlines when the princess said "there were three of us in this marriage", referring to Prince Charles' relationship with Camilla Parker-Bowles. At the time Princess Diana was separated from Prince Charles but not yet divorced.
Earlier this month the Daily Mail published notes Earl Spencer says he made with Bashir two months before the interview.
Our correspondent Jonny Dymond said the notes appeared to record Bashir "spinning lie after lie about members of the Royal Family, and its staff, in an attempt, Earl Spencer says, to win his trust and that of his sister".
These claims, described by the Mail as "preposterous lies", include that Diana's private correspondence was being opened, her car tracked and phones tapped.
It was also claimed that her bodyguard was plotting against her and close friends were betraying her by leaking stories to the press.
The Princess of Wales died on 31 August 1997, aged 36, in a car crash in a Paris underpass.
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