The husband of Nazanin Zaghari-Ratcliffe, the British-Iranian dual national held in Tehran on spying charges, says he has been given hope by Iran’s offer of a prisoner swap to secure her release.

Iran’s foreign minister, Mohammad Javad Zarif, said on Wednesday he had the power to authorise Zaghari-Ratcliffe’s release in return for the freeing of an Iranian woman held in Australia.

Richard Ratcliffe was initially sceptical of the offer but speaking on Thursday he was more positive, describing it as a “good thing”.

“It is definitely hopeful,” he told BBC Radio 4’s Today programme. “That’s the first time the foreign minister has talked so explicitly about Nazanin being released. He did link her to a prisoner swap which was the first time we’ve heard [that, and it] was very surprising.”

British diplomats are expected to be cautious about the Iranian offer, especially if it involves an admission that Zaghari-Ratcliffe is guilty of espionage, the charge on which she has been found guilty by the Iranian courts. There is also an issue of trust between the two countries, and a concern that the swap might fall apart if Britain agrees to the deal.

There is also an anxiety that the move involves an acceptance that there is a parallel between the Australian case and that of Zaghari-Ratcliffe, including possibly an admission that the Australian judicial system is not capable of dealing with the case fairly.

It is understood that the Iranians have referred to the Australian case as an injustice in private conversations, but not explicitly as part of a prisoner exchange.

On the other hand, Britain is likely to regard it as significant that the Iranian foreign minister said he was authorised to make the offer. In the past the UK foreign secretary, Jeremy Hunt, was told the issue was purely a matter for the Iranian judiciary.

Hunt was unable to meet the justice minister during a visit to Tehran.

Some observers said the offer was likely to be seen as primarily an attempt to show a softer Iranian side to an American audience after the Trump administration’s decision this week to tighten the economic squeeze on Tehran by removing waivers from sanctions on those countries still permitted by the US to buy Iranian oil.

On Wednesday, Ratcliffe said he was unsure if the proposal was serious and that it had left him “disoriented”. He told Today the family felt “blindsided” by the swap proposal.

He said: “We’ve been scrambling a bit to work out what it means and what’s going on. We’ll need to talk to the Foreign Office to see what they make of it. But the basics of him signalling that she could be released … is of course a good thing.

“It is a bit unclear what he means by it, but he was very specific in saying Nazanin and linking her to prisoner swap with someone held in Australia and also mentioning that he had the authority to negotiate these things.”

Facebook Twitter Pinterest Richard Ratcliffe delivers a Mother’s Day card and flowers to the Iranian embassy in Knightsbridge, south-west London. Photograph: Will Oliver/EPA

Zarif made the offer at the Asia Society in New York. He said: “We hear about Nazanin Zaghari[-Ratcliffe] and her child, and I feel sorry for them and I have done my best to help, but nobody talks about this lady in Australia … I put this offer on the table publicly now. Exchange them. I’m ready to do it. And I have authority to do it.”

He has previously said the issue is for the Iranian judiciary and rejected any British representations on Zaghari-Ratcliffe’s behalf, arguing it does not recognise dual national status.

In March, Hunt escalated the dispute by granting Zaghari-Ratcliffe diplomatic status.

Ratcliffe said the move had some positive effects: “We were completely blindsided by what happened yesterday. Does it directly link [to diplomatic status]? Well, maybe, is the honest answer.” He said his wife often felt subject to diplomatic game playing.

“Certainly her family were fairly downbeat yesterday … We have had a number of times when we have felt close to [her] being released and then we haven’t been. There are lows that follow it,” Ratcliffe said.

Zaghari-Ratcliffe went on hunger strike for three days in January in an attempt to improve her treatment. Ratcliffe said: “What we’ve being saying to the government is she won’t go on for much longer without her again taking action into her own hands. So please push, please do what you can and let’s hope we don’t come to where we might need to get to.”