British foreign minister Dominic Raab said the second trial was "unacceptable" and called on Iran to let Zaghari-Ratcliffe return to Britain.
"She is a political hostage and I suspect the Iranians are just trying to put pressure on the British government and use her for political leverage".
Mrs Zaghari-Ratcliffe's family fear the court case could be used to block her return to the United Kingdom because the Iranian government wants to keep her for diplomatic leverage.
"It is unacceptable that Iran has chosen to continue the second case against Nazanin Zaghari-Ratcliffe", Raab said in a statement on Sunday.
Zaghari-Ratcliffe's yearslong case has sparked global outrage and strained ties between Britain and Iran.
Legally, the court should announce the verdict in a week but it is up to the judge and her lawyer says he is hopeful that she will be acquitted.
Earlier, Mr Ratcliffe said his wife had been due to report to court at 09:00 Iranian time (05:30 GMT), but did not know when her trial would be heard, what it would entail or how long it would be.
Sunday's hearing, which lasted just over 20 minutes, was a continuation of the trial that was adjourned in November, on charges originally brought in 2017.
She completed the latter stages of her sentence under house arrest, living with her parents in Tehran, and had her ankle tag removed on Sunday - but she is now facing fresh charges of spreading propaganda against the Iranian regime.
An undated handout image released by the Free Nazanin campaign in London on June 10, 2016 shows Nazanin Zaghari-Ratcliffe (L) posing for a photograph with her daughter Gabriella.
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The British government has previously admitted it owes Iran up to $390 million (£300 million), but both countries have denied any link with the Zaghari-Ratcliffe case.
Rebecca added: "At the outset, a five-year sentence was our worst-case scenario, and Nazanin has been counting down to her release date".
He told the PA news agency: "I think the judge saying this is the last trial is just a nice feeling - I think it is a bit like you've done an exam, it was frightful, you don't know whether you passed or not, but at least it is done and there's just a relief that comes with that".
While in prison, she suffered from lack of hygiene and even contemplated suicide, according to her husband.
Mr Ratcliffe said he was feeling "better than I was expecting" but was still "guarded, cautious and worried".
Rights groups accuse Iran of using dual-national detainees as bargaining chips for money or influence in negotiations with the West, something Tehran denies.
He said he had no new information from the Iranian authorities or the Foreign Office in London.
Mr Ratcliffe told the PA news agency: "The Revolutionary Court is not in the business of acquitting people, it does only do convictions but it can take its time in doing that and the sentence can vary".
Mrs Zaghari-Ratcliffe was held for years in the notorious Evin prison, where she was interrogated while blindfolded.
Mr Ratcliffe had hoped the British ambassador would accompany his wife to the trial, but said the British embassy in Tehran had declined.