UK-Iranian dual national Nazanin Zaghari-Ratcliffe is on her way back to Britain after being detained by Tehran nearly six years ago, in a move that could boost western diplomatic efforts to revive the 2015 nuclear accord.

Zaghari-Ratcliffe, who was arrested on spying charges in 2016, is expected to land in the UK with Anoosheh Ashoori, another newly-released Iranian-British national, late on Wednesday evening.

Britain had also settled $530mn of debt owed to the republic for the purchase of Chieftain tanks that Tehran ordered before the 1979 Islamic revolution.

Liz Truss, UK foreign secretary, said the debt had been settled “in full compliance with UK and international sanctions and all legal obligations”. She added that the “funds will be ring-fenced solely for the purchase of humanitarian goods”.

Richard Ratcliffe had long believed that his wife’s continued detention was linked to a number of issues, including the outstanding debt related to the tanks that were not delivered after the last shah was ousted in the revolution.

The UK government has always insisted that the two issues were not linked. Truss said on Twitter: “I can confirm Nazanin Zaghari-Ratcliffe and Anoosheh Ashoori will return to the UK today, and Morad Tahbaz has been released from prison on furlough. They will be reunited with their families later today.

“We will continue to work to secure Morad’s departure from Iran.”

Zaghari-Ratcliffe’s local MP, Tulip Siddiq, shared an image of her constituent on a plane flying out of Tehran. “It’s been six long years — and I can’t believe I can FINALLY share this photo,” she said. “Nazanin is now in the air flying away from six years of hell in Iran.”

Prime Minister Boris Johnson said: “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.

“The UK has worked intensively to secure their release and I am delighted they will be reunited with their families and loved ones.”

Ashoori, 67, was arrested in 2017 and sentenced to 12 years in jail on espionage charges, which included “cooperating with a hostile state against the Islamic Republic” and “obtaining illicit funds” amounting to €33,000.

Morad Tahbaz, 66, a businessman and environmental activist, holds Iranian, American and British nationalities. He was arrested in 2018 along with eight other environmental activists. A co-founder of the Persian Wildlife Heritage Foundation, Tahbaz was sentenced to 10 years in prison for collaboration with a hostile government, the US.

The releases come as western powers are hoping to secure a deal with Iran to revive the 2015 nuclear accord Tehran signed with world powers.

Zaghari-Ratcliffe’s release would remove a longstanding point of contention between the UK and the Islamic republic.

Negotiators at nuclear talks in Vienna have for weeks been saying they were closing in on a deal that would lead to the US rejoining the accord abandoned by former US president Donald Trump four years ago. The Biden administration would then provide sanctions relief to Iran, in return for it reversing its nuclear activity to agreed limits.

The UK is a signatory to the 2015 accord, alongside France, Germany, Russia and China, and heavily involved in EU-brokered talks in Vienna to save the agreement.

US officials have previously said that the release of US-Iranians held in the republic would also be key to any deal to revive the 2015 nuclear accord.

The Vienna talks stalled last week after Moscow demanded it would need guarantees from the US that sanctions imposed on Russia after President Vladimir Putin invaded Ukraine would not impede its trade with Iran.

The move forced EU mediators to pause the indirect talks between the US and Iran in Vienna just as negotiators hoped to push them over the finish line. But on Tuesday, Moscow suggested it would not scupper the process, saying Washington had provided the assurances it was seeking and that the final details of an agreement to revive the deal were being “polished”. 

After Iran signed the accord in 2015, it transferred enriched uranium to Russia to reduce its stockpile and it would have to take similar action if an agreement was reached with the Biden administration.