Britain is preparing plans to transfer over £400 million to Iran to secure "goodwill", the Telegraph understands, as the Foreign Secretary vowed to do "everything" he can to release Nazanin Zaghari-Ratcliffe.

Officials have sought legal advice regarding US and United Nations sanctions which have so far prevented them from handing over the money, the result of a controversial 1970s arms deal, amid claims the British mother is being held as "collateral".

It came as an Iranian news agency reported that Mr Johnson is due to visit the country next week.

Senior Whitehall sources have told The Telegraph that while the Government is at pains to ensure the payment is not directly linked with Ms Zaghari-Ratcliffe's release, work has intensified in recent months in a bid to improve relations with Iran.

Diplomats believe handing over the money, which is being held in a frozen account by the courts and which Britain agrees should be returned, could help speed up her release from jail.

It came after Boris Johnson met Ms Zaghari-Ratcliffe's husband for the first time, where they agreed that lawyers would meet in the coming weeks to discuss the possibility of diplomatic protection for her.