Iran has claimed the UK paid over $530million (£400million) before the release of Nazanin Zaghari-Ratcliffe, it has been reported by Reuters.
The release of Mrs Zaghari-Ratcliffe and fellow detainee Anousheh Ashoori was announced this morning.
She will return to the UK this evening on a private Government chartered Titan Airways Boeing from Muscat to RAF Brize Norton, landing just after 10.30pm, The Times understands.
The British-Iranian national was detained in Iran for six years after being arrested in Tehran airport in April 2016 on charges of plotting to overthrow the Iranian government.
According to her family, she was told by authorities that she was being detained because of the UK's failure to pay their outstanding debt of £400m to Iran.
This debt is related to the UK cancelling their order for 1,500 Chieftain tanks in the 1970s following the Islamic revolution.
Nazanin Zaghari-Ratcliffe and her daughter Gabriella
KARL BRANDT/FREE NAZANIN CAMPAIG
Richard Ratcliffe, husband of Nazanin Zaghari-Ratcliffe has been campaigning for his wife's release since her arrest
PETER CZIBORRA
Following the announcement of Mrs Zaghari-Ratcliffe's release, Former Foreign Secretary Sir Malcolm Rifkind told GB News that the reported payment is "not blood money".
He said: "It's not payment for blackmail...We’ve only paid what the Government acknowledged quite some time ago.
"Apart from the politics and all the serious and difficult issues that had to be reconciled, this is a human story.
"This is a woman who has been deprived of her family, of her child for six years now never knowing how long this would last."
Late last year, Foreign Office Minister James Cleverley told MPs the Government accepted that money to Iran was owed, but that settling it was "not easy".
Mr Cleverley said in the House of Commons: "The UK Government recognise that we have a duty to legally repay this debt and we continue to explore all legal options to resolve this 40-year-old case."
But he added: "We do not accept British dual nationals being used as diplomatic leverage."