The Iranian foreign minister has claimed that the UK offered release £400m owed to Iran from a decades-old deal in return for Tehran freeing Nazanin Zaghari-Ratcliffe, the British-Iranian charity worker.

Speaking at the UN General Assembly in New York, Javad Zarif said that both Philip Hammond and Boris Johnson were open to the proposal when they were foreign secretary but that it was taken off the table by Jeremy Hunt.

The idea involves the UK returning £400m which the Shah of Iran paid in the 1970s to purchase 1,750 tanks and armoured cars from Britain. The UK did not deliver the tanks after the Shah was overthrown and Iran has repeatedly demanded the money back.

Mr Zarif said both Mr Hammond and Mr Johnson considered the idea but that it was nixed by Mr Hunt because he harboured ambitions to become prime minister and did not want to be seen to be paying a ransom to Iran.

“Jeremy started talking about ‘you’re asking for ransom’. And I said: ‘Come on, this is not something that I started. This is something that Philip started,’” Mr Zarif said. “Jeremy started to play tough because he wanted to become prime minister and it didn’t work out for anybody.”