British-Iranian aid worker Nazanin Zaghari-Ratcliffe and dual national Anoosheh Ashoori returned to Britain from Iran on Thursday, ending a long ordeal where they essentially became a bargaining chip in the West's talks with Iran over its nuclear program.

They arrived at a British military airbase in Oxfordshire just after 1 a.m. local time, walking off the plane together, smiling and waving as they entered an airport building.

"It has been a really difficult 48 hours," British Foreign Secretary Liz Truss said after the arrival. "The expectation was that they would be released but we weren't sure right until the last minute so it's been very emotional but also a really happy moment for the families."

Prime Minister Boris Johnson celebrated the news on Twitter earlier in the day.

"I am very pleased to confirm that the unfair detention of Nazanin Zaghari-Ratcliffe and Anoosheh Ashoori in Iran has ended today, and they will now return to the UK," Johnson said in a tweet.

Zaghari-Ratcliffe's husband Richard said the struggle to bring back his wife appeared to finally be over. "It's just a relief, the idea that we can go back to being a normal family, that we don't have to keep fighting, that this long journey is almost over," he said outside his London home ahead of his wife's landing.

The dual citizen was arrested by Revolutionary Guards at Tehran airport in 2016, while trying to return to Britain with her then 22-month-old daughter Gabriella after visiting her parents.

She was later convicted by an Iranian court of plotting to overthrow the clerical establishment. Her family and the foundation denied the charge.

Ashoori was sentenced to 10 years in jail in 2019 for spying for Israel's Mossad and two years for "acquiring illegitimate wealth", Iran's judiciary ruled.

Ashoori's family thanked those who had worked towards his release. "1,672 days ago our family's foundations were rocked when our father and husband was unjustly detained and taken away from us," they wrote in a statement:

"Now, we can look forward to rebuilding those same foundations with our cornerstone back in place."

Nuclear deal

In February, as months of talks on reviving a 2015 nuclear deal with Iran came closer to fruition, Tehran said it would release the prisoners in return for the unblocking of frozen assets and the release of Iranians held in Western jails.

The nuclear talks were close to a deal until last-minute Russian demands for guarantees loosening some of the sanctions imposed amid its military operation in Ukraine derailed the talks.

Russia has now toned down its demands to cover only work linked to the nuclear deal, which leaves only a small number of details to be resolved between Washington and Tehran.

Separately, Britain said detained Iranian-American environmentalist Morad Tahbaz, who also holds British citizenship, had been released on furlough on Wednesday.

Historic debt

Iran's semi-official Fars news agency said Zaghari-Ratcliffe and Ashoori were freed after Britain repaid a historic debt.

Iran's clerical rulers say Britain owed Iran 400 million pounds ($520 mln) that Iran's former monarch, the Shah, paid up front for 1,750 Chieftain tanks and other vehicles, almost none of which were delivered after the Islamic Revolution of 1979 toppled the U.S.-backed leader.

Truss said Britain had been looking at ways to pay the debt.

"We have the deepest admiration for the resolve, courage and determination Nazanin, Anoosheh and Morad, and their families, have shown. They have faced hardship that no family should ever experience and this is a moment of great relief," she said in a statement.

"In parallel, we have also settled the IMS debt, as we said we would," she added, referring to the debt for military equipment. She said the debt had been settled in full in compliance with international sanctions on Iran and the funds would be ring-fenced for buying "humanitarian goods."

Iran's top diplomat Hossein Amirabdollahian on Wednesday said Britain had paid its debt a few days ago, denying any links between the payment and the release of the prisoners.