Nazanin Zaghari-Ratcliffe has said she was forced to sign a “false confession” in front of a UK government witness before she was allowed to leave Iran.
The British-Iranian hostage survivor described the act, which was captured on camera, as “dehumanising”.
She said she expects Tehran to use it against her in the future.
While “under duress”, the 44-year-old charity worker claimed she was forced to admit to spying allegations made by Iran after they detained her for six years – a charge she and the UK denied.
She said she was taken to the airport by the Revolutionary Guards without seeing her parents on the day in March when she was to be freed.
“Instead I was made to sign the forced confession at the airport in the presence of the British government,” Ms Zaghari-Ratcliffe told the BBC.
She was then told by the Iranians that the UK had settled a historic £400m debt dating to the 1970s.
“They told me that ‘you won’t be able to get on the plane’. And I knew that that was like a last-minute game because I knew they were… they told me that they have been given the money,” she said.
“So what is the point of making me sign a piece of paper which is incorrect? It’s a false confession.”
Mrs Zaghari-Ratcliffe also told the BBC’s Emma Barnett that a British official was present at the time she signed the document.
“The whole thing of me signing the forced confession was filmed,” she said.
“It’s a tool. So I’m sure they will show that someday.”
The revelation comes after her husband Richard Ratcliffe alluded to “mistakes made at the end” of the ordeal in Iran.
Speaking earlier this month after his wife’s first meeting with Prime Minister Boris Johnson since her release, Mr Ratcliffe said: “I think there are lessons to learn, there is a wider problem.
“We talked about the mistakes made at the end. It was rough at the end, and I think, when Nazanin is ready to talk about it, that is something that we need to go through.”
Meanwhile, at a press conference following her release in March, Mrs Zaghari-Ratcliffe said it had taken the government far too long to pay the multimillion-pound debt to Iran, which helped secure her release.
She said although she could not be happier to be home, “this should have happened six years ago”.
Following the remarks, she received significant backlash online from people saying she should be grateful, but Downing Street was quick to defend her.
Boris Johnson’s official spokesman said: “Clearly someone who has been through something like Mrs Zaghari-Ratcliffe has should not get any abuse.
“As a UK citizen, she is rightly able to voice her opinion on any topic she wishes.”