Nazanin Zaghari-Ratcliffe, an Anglo-Iranian charity worker, has been sentenced to 12 months in prison in Iran and banned from leaving the country for a year after being convicted of “propaganda against the [Islamic Republic’s] establishment”.

The ruling comes a month after Zaghari-Ratcliffe, 42, was released after serving five years for espionage, the last of which was spent under house arrest in Tehran.

Her husband, Richard Ratcliffe, has said he believes his wife’s continued detention is linked to the UK’s unresolved £400m debt to Iran for tanks that were paid for but not delivered.

Zaghari-Ratcliffe’s lawyer, Hojjat Kermani, told the Financial Times that the latest verdict was issued by the Revolutionary Court, which deals with high-level security cases, and that he was informed of it on Monday morning.

The charge is related to Zaghari-Ratcliffe’s alleged participation in a protest outside the Iranian embassy in London in 2009 when the country went through some of its biggest anti-regime protests following a disputed presidential election.

Zaghari-Ratcliffe has 20 days to appeal against the verdict, Kermani said, adding: “For now, she is free and is at home but cannot leave the country.”

UK prime minister Boris Johnson described the verdict and sentence as “cruel, inhumane and wholly unjustified”.

He said the government would “redouble our efforts” to secure her release, and was co-ordinating with the US on the matter.

“I don’t think it is right at all that Nazanin should be sentenced to any more time in jail,” Johnson said. “I think it is wrong that she is there in the first place, and we will be working very hard to secure her release from Iran, her ability to return to her family here in the UK — just as we work for all our dual national cases in Iran.”

The verdict comes a week after a high court hearing in London was postponed which further delayed the resolution of the UK debt to Iran over London’s failure to deliver Chieftain tanks bought before the 1979 Islamic revolution.

Both governments insist there is no link between the two matters.

Lisa Nandy, the shadow foreign secretary, described the news as “absolutely devastating”, adding: “For more than five years, Nazanin’s freedom has been used as a political bargaining chip that has resulted in an unimaginable ordeal for her and her family.

“The UK government has serious questions to answer over their failed strategy to bring her home and the foreign secretary must come to parliament to explain what actions he will take to ensure Nazanin is returned home to her family.”

Zaghari-Ratcliffe was jailed in Iran five years ago on spying charges that she has always denied. She was moved to house arrest in Tehran in March last year because of the pandemic until her sentence ended last month and her ankle tag was removed.

However, she was not allowed to leave the country because of the new charges.

Although Iran does not recognise its citizens’ dual national status and says such judicial matters are domestic affairs, the Islamic republic has a record of releasing dual nationals in exchange for money or swapping them for Iranian prisoners held overseas.

Western diplomats in Tehran say the US administration of President Joe Biden has focused on the release of a few Iranian-Americans, including businessman Siamak Namazi, and his 84-year-old father Baquer Namazi, in Iranian jails.

In the latest round of efforts to revive the Iran nuclear deal in Vienna, in which the US is indirectly represented, western diplomats in Tehran said Iran was reminded of the need to release these dual-national prisoners before any nuclear-related agreement could be reached.

It is not clear whether Iran sees the American prisoners as leverage to gain access to some of its money in overseas banks frozen by US sanctions or would like to exchange them with Iranians in US jails accused of circumventing sanctions.

Some Iranian-European nationals are also in jail for whom Iran is suspected to be seeking prisoner swaps.