Nazanin Zaghari-Ratcliffe has said Iran forced her to sign a last-minute false confession at the airport as a condition of her release.
She told the BBC a UK official was with her when she signed the statement "under duress" before the Iranian authorities would let her fly home.
Mrs Zaghari-Ratcliffe was released in March, six years after being arrested on spying charges.
She said Iran extracts false confessions as a propaganda tool.
Speaking to the BBC's Emma Barnett for Woman's Hour, Mrs Zaghari-Ratcliffe also questioned why the UK did not challenge Iran over forcing her to admit to crimes she did not commit as a condition of her release.
She said she was taken away by Iranian Revolutionary Guards without seeing her parents and "made to sign the forced confession at the airport in the presence of the British government".
Mrs Zaghari-Ratcliffe said was told she would not be allowed to get on the plane back to the UK without signing the confession, adding that Iranian officials filmed the whole process.
The former political prisoner said she wanted to make sure people knew she had been forced to sign, to prevent the Iranian regime from exploiting her "dehumanising" confession.
"Why would I sign something? I have been trying very, very hard for the past six years to say I have not done it," she said.
"All the false confessions that we have been exposed to, they have no value.
"They are just propaganda for the Iranian regime to show how scary they are and they can do whatever they want to do."