Negotiations with Tehran to free British-Iranian mother Nazanin Zaghari-Ratcliffe are “moving forward” and are “going right up to the wire”, Boris Johnson has said.

The Prime Minister cautiously raised hopes on Wednesday that the dual national’s six-year ordeal could come to a close after suggestions the mother-of-one has had her passport returned.

But Mr Johnson, during a trip to the Middle East, was careful not to elaborate further on the state of negotiations with Tehran “because those negotiations continue to be under way”.

A glimmer of optimism for the 43-year-old came a day earlier when her constituency MP in Hampstead and Kilburn, Tulip Siddiq, said her British passport had been returned.

Mrs Zaghari-Ratcliffe’s family, however, were keeping their hopes cooled, having experienced numerous setbacks and disappointments during the saga.

Mr Johnson confirmed a British negotiating team was working in Tehran to secure the release of dual nationals, while Mrs Zaghari-Ratcliffe remains at her family home in the Iranian capital.

“I really don’t think I should say much more, I’m sorry, although things are moving forward,” he told broadcasters at the Emirates Palace hotel in Abu Dhabi.

“I shouldn’t really say much more right now just because those negotiations continue to be under way and we’re going right up to the wire.”

Mrs Zaghari-Ratcliffe’s sister-in-law Rebecca Ratcliffe said the family are trying not to raise their hopes too much.

She told BBC Breakfast: “This may all be over in a week or two. But also it may just be a stunt from the Iranians. We’ve had this before.

“We’ve had many ups over the last six years and been told she’s been about to be released. So there’s an element of false hopes and I think our family, Nazanin, her parents, find it hard to get too excited at the moment.”

Mrs Zaghari-Ratcliffe was arrested in April 2016 as she prepared to fly back to the UK, having taken her daughter Gabriella – then not even two years old – to see relatives.

She was accused of plotting to overthrow the Iranian government and sentenced to five years in jail, spending four years in Tehran’s Evin Prison and one under house arrest.

Both the British Government and Mrs Zaghari-Ratcliffe have always denied the allegations.

While the details of the negotiations remain unclear, it is possible they are linked to a £400 million debt dating back to the 1970s owned to Iran by the UK.

The Government accepts it should pay the “legitimate debt” for an order of 1,500 Chieftain tanks that was not fulfilled after the shah was deposed and replace by a revolutionary regime.

Foreign Secretary Liz Truss told Sky News on Wednesday that it is a “priority to pay the debt that we owe to Iran”.

Tehran remains under strict sanctions, however, which have been linked to the failure to clear the debt.

Ms Siddiq said Mrs Zaghari-Ratcliffe is “waiting at home (in Iran), anxiously, for a knock on the door, with her bags packed”.

The Labour MP told LBC: “I’ve been speaking to her husband regularly, Richard Ratcliffe, who, as you know, has been fighting a relentless campaign on her behalf. And he said to me that past prisoners from Evin prison in Tehran have gone to a third country, usually maybe to Oman, or to another country, where a prisoner is handed over and then made their way over to whichever the country of origin is, whether it’s Britain or Australia or America.

“I think he is anticipating his wife will probably be handed over to another country and then will eventually make her way to the UK. What I can’t tell you is whether it’s going to be tomorrow or the end of the week, or who knows?”