UK Prime Minister Boris Johnson has demanded Iran's President Hassan Rouhani release all dual-nationals, including Nazanin Zaghari-Ratcliffe.

She faces further court action despite having served her five-year sentence on charges of plotting against the Iranian government.

"The prime minister raised the case of Nazanin Zaghari-Ratcliffe and other British-Iranian dual nationals detained in Iran and demanded their immediate release," according to a Downing Street spokesperson.

"He said that, while the removal of Nazanin Zaghari-Ratcliffe's ankle monitor was welcome, her continued confinement remains completely unacceptable and she must be allowed to return to her family in the UK," the spokesperson added.

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Zaghari-Ratcliffe's sentence ended on March 7. She has always maintained her innocence and said she was only in the country to visit family with her newborn child Gabriella. 

The former aid worker, 42, had an ankle tag removed and was allowed to leave home detention to visit relatives in Tehran on the day she was supposed to be freed.  

Britain has called for her to be able to return to her family in the UK.

But she now faces another court appearance in Iran next Sunday, confounding hopes among her family, friends and supporters of an immediate return home.

Downing Street said Johnson had told Rouhani "while the removal of Nazanin Zaghari-Ratcliffe's ankle monitor was welcome, her continued confinement remains completely unacceptable."

On Sunday, Zaghari-Ratcliffe's husband, Richard Ratcliffe, said she was "genuinely happy" to have been given a greater degree of freedom but he cautioned she "remained in harm's way" with the fresh court summons hanging over her.

UK 'committed' to Iran nuclear deal

Johnson and Rouhani also discussed negotiations to resurrect Iran's 2015 nuclear deal with world powers. 

Britain has remained one of the signatories to the deal, known as the Joint Comprehensive Plan of Action (JCPOA), despite the U.S.'s withdrawal under former president Donald Trump in 2018.

"The Prime Minister also stressed that while the UK remains committed to making the Iran nuclear deal a success, Iran must stop all its nuclear activity that breaches the terms of the JCPOA and come back into compliance," his office said.

Adding: "He stressed the importance of Iran seizing the opportunity presented by the United States' willingness to return to the deal if Iran comes back into compliance."

Britain, along with France and Germany, has criticized Iran for failing to comply with the nuclear deal and grant access to the United Nations' nuclear watchdog to its sites.

U.S. President Joe Biden has signaled a readiness to return to the nuclear deal.

But Washington said on Wednesday it would not look to revive the accord before Iranian elections in June, which are expected to fall in the favor of a more hardline president in Tehran.