The group's plane touched down in Qatar's capital shortly after 3.30pm UK time following their departure from Tehran. It is thought they boarded another plane in Doha and are expected to arrive in the US later on Monday night.

The British man freed is environmentalist Morad Tahbaz, 67, who has British, US and Iranian citizenship.

He was among the dual nationals being held at the time negotiations were under way involving the UK government to free former detainee Nazanin Zaghari-Ratcliffe.

The group was joined on the flight by two family members and the Qatari ambassador to Iran, a source told Reuters.

The $6bn (£4.8bn) funds, once frozen in South Korea, were released in Qatar after the Biden administration issued a waiver for international banks to transfer frozen Iranian money without the repercussions of US sanctions, an Iranian official announced on state television.

The deal also included the release of five Iranian citizens held in the US. Two of them, named in Iranian media reports as Mehrdad Moin-Ansari and Reza Sarhangpour-Kafrani, landed in Tehran on Monday after also stopping over in Qatar.

Jailed at notorious prison

The US-Iranian dual nationals released by Iran include businessmen Siamak Namazi, 51, and Emad Shargi, 58.

Together with Mr Tahbaz, they had all been jailed at the notorious Evin Prison in Tehran on spying charges.

In a statement upon landing in Doha on Monday, Mr Namazi thanked everyone who "didn't allow the world to forget me" after spending 2,898 days behind bars.

"For almost eight years I have been dreaming of this day. Now that it is finally here, I find my ineffable joy of my forthcoming reunification with my family is laced with sorrow - a painful and deep feeling of guilt for taking my breaths in freedom while so many courageous individuals that I love and admire continue languishing behind those walls," he said.

"They are detained for demanding the dignity and freedom that every human being is inherently entitled to; for reporting the truth; for worshipping their God; for being a woman. For nothing."

He added: "What I want more than anything is assurance that no one else will know the interminable anguish that my family and I experienced. But sadly, many are suffering those miseries right now."

The identity of the fourth and fifth prisoners to be released has not been made public.

As well as the two Iranian former prisoners who have arrived in Tehran after being freed by American officials, two others will stay in the US and one detainee will join his family in a third country, an Iranian government spokesperson said.

Iran's foreign ministry spokesperson Nasser Kanaani said earlier: "The issue of swap of prisoners will be done on this day and five prisoners, citizens of the Islamic Republic, will be released from the prisons in the US.

"Five imprisoned citizens who were in Iran will be given to the US side reciprocally, based on their will. We expect these two issues [to] fully take place based on agreement."

It comes weeks after Iran said the five were released from prison and placed under house arrest.

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British national among those being released

London-born Mr Tahbaz was arrested in 2018 and sentenced to 10 years in prison for "assembly and collusion against Iran's national security" and working for the US as a spy.

He has - at various stages - been the subject of ongoing negotiations for his release involving British authorities, including when Dominic Raab and Liz Truss were foreign secretaries.

He was temporarily released on the same day charity worker Ms Zaghari-Ratcliffe and retired civil engineer Anoosheh Ashoori were freed, but was later returned to custody.

His daughter Roxanne had been among those calling on the UK government to do more to get him released.

He is a prominent conservationist and board member of the Persian Wildlife Heritage Foundation, which seeks to protect endangered species.

Mr Namazi was convicted in 2016 of espionage-related charges the US has rejected as baseless and sentenced to 10 years in prison.

Mr Shargi was convicted of espionage in 2020 and also sentenced to 10 years in prison.

The money from South Korea represents funds Seoul owed Iran, but had not yet paid, for oil purchased before Donald Trump's administration imposed sanctions on such transactions in 2019.

The US maintains the money will be held in restricted accounts in Qatar and will only be able to be used for humanitarian goods such as medicine and food - transactions allowed under American sanctions targeting the Islamic Republic over its advancing nuclear programme.

The West has accused Iran of using foreign prisoners as bargaining chips, an allegation Tehran rejects.

It came as Iran's president Ebrahim Raisi arrived in New York on Monday for a series of events, including a meeting of the United Nations General Assembly.

The UK's foreign secretary James Cleverly told Sky News it was right that the leader was coming so that he could be "held to account".

The president will also attend an event hosted by thinktank the Council of Foreign Relations, which has invited him to speak on Tuesday. Mr Cleverly will appear at the same function.

But critics on social media have said Mr Raisi should have been met with handcuffs in New York, rather than speaking invitations, because of his government's oppression of a year of protests in Iran.