Downing Street also said Tehran should “permanently” release the north London mother-of-one.
Mrs Zaghari-Ratcliffe has said she “really can’t take it any more” after a second court case against her in Iran was postponed at short notice with no explanation and no future date set.
The 42-year-old has been detained in Iran since 2016 when she was sentenced to five years in prison over allegations, which she strongly denies, of plotting to overthrow the Iranian government.
Earlier this year, she was granted temporary leave from prison because of the Covid-19 outbreak and has been living at her parents’ house in Tehran with an ankle tag.
Her husband Richard Ratcliffe said his wife was expecting to be tried yesterday on fresh charges of “spreading propaganda against the regime”.
But Mrs Zaghari-Ratcliffe was not taken to the Revolutionary Court and the case against her was postponed.
In London, the Prime Minister’s official spokesman said: “It’s a groundless court case.
“What Iran needs to do is permanently release Nazanin and allow her to return to her family in the UK.”
Mrs Zaghari-Ratcliffe told her husband on Sunday morning of the “level of stress” she was having to endure.
“People tell me to calm down. You don’t understand what it is like. Nothing is calm,” she said.
“This morning I just wanted to scream out loud for 10 minutes or to bang my head against the wall – just to let it out.
“I really can’t take it any more. They have all these games and I have no power in them.
“Sometimes I am just full of anger ready to explode. I find myself hating everything in this life, including myself. There is no escape.
“I would have rather it happened today. I do not sleep at all while the case is hanging over me.
“This morning I wanted to get it over with – to know where I stand now rather than continue with this whole stupid game.”
Having been moved to house arrest in March, when thousands of prisoners were granted clemency and released from Iranian jails amid the Covid-19 outbreak, Mrs Zaghari-Ratcliffe was returned to court on Tuesday only months from her expected release date and told she would face a second trial.
Mr Ratcliffe said neither his wife nor her lawyer were given any explanation about why or when the trial might be rescheduled.
He said: “It is too early to say what the postponement means, except that this remains a game of cat and mouse between governments, with us living life as a piece of bait.
“For Nazanin, the uncertainty remains deeply traumatic as we await the next move.
“As we do so, the importance of the Government’s diplomatic protection grows.”
Amnesty International UK accused Iranian authorities of “playing cruel political games” with Mrs Zaghari-Ratcliffe and called on the UK Government to make it an “absolute priority” to get her home for Christmas.
It has been claimed Mrs Zaghari-Ratcliffe is being held to force the UK to settle on a debt dating back to the 1970s when the then-shah of Iran paid the UK £400 million for 1,500 Chieftain tanks.
After he was toppled in 1979, the UK refused to deliver the tanks to the new Islamic republic and kept the money, despite British courts accepting it should be repaid.
Leanna Burnard, legal officer at Redress, which has acted as legal representatives for Mrs Zaghari-Ratcliffe and her husband, said: “The complete absence of the rule of law in this case ensures that Nazanin is held in a constant state of stress, not knowing if she might be returned to prison or when she might see her husband and daughter again.”