The husband of a British-Iranian woman jailed in Tehran has urged Boris Johnson to pay a £400 million debt Britain owes Iran as concerns mount for the future of his wife and other British citizens in detention in the country.

Richard Ratcliffe says his wife Nazanin Zaghari-Ratcliffe now fears receiving a second jail sentence as Iran seeks revenge on the west over the assassination of General Qassem Suleimani by the US last week.

Mrs Zaghari-Ratcliffe is midway through a five-year sentence, officially on spying charges. The 40-year-old is among as many as five people with dual British-Iranian nationality, or with UK connections, believed to be in prisons in Iran at present.

Britain has owed the £400 million since pre-revolutionary Iran paid it for 1,500 Chieftain army tanks in the 1970s. The deal was cancelled after the Shah of Iran was deposed in 1979, but while Iran has demanded the money back, Britain has so far refused.

"When he was Foreign Secretary, Mr Johnson indicated that this debt would be paid to secure the release of Nazanin and other British hostages," Mr Ratcliffe wrote in the Daily Mail.

"So far he has not kept his promise. He talks of protecting British interests, seemingly shipping routes and oil. That's all very well. But what about British citizens? What about protecting their lives?"

Mr Ratcliffe said he was at least pleased to have had his and his wife's daughter Gabriella back at his side since she was allowed to leave Iran last October. The five-year-old will have her first full day of school today.

"Nazanin would have wanted so desperately to be there for her daughter's first day, to brush her hair, help with her uniform, take her into the classroom and give her a hug," Mr Ratcliffe wrote.

"I am urging the Prime Minister to show real leadership and ensure this crisis doesn't get worse, for the sake of my wife and the other Western detainees illegally held in Iran."