"We are strong supporters" of the nuclear deal, said the official, asking not to be named. "I don't think you will find any change in the British government position."

Johnson is expected at the G7 in Biarritz to hold talks with US President Donald Trump, who pulled out of the landmark accord last year.

Trump and Johnson have a strong personal rapport and the British premier, who took office last month, has emphasised the importance of close relations with the United States as Britain prepares to leave the European Union.

But the British official said the nuclear deal, which Britain helped negotiate, is important to ensure that Iran never obtains an atomic weapon. Tehran insists its nuclear programme is peaceful.

"It is important that it (the deal) continues," the official said.

"I think on this issue there is a sort of expectation that we are going to have a meeting with the American president and our position will change. But our position on Iran is well known."

If the US president has other ideas "we are very happy to talk about them", the official said, adding that for now the nuclear deal was the "best way" of preventing Iran from obtaining a nuclear weapon.

French President Emmanuel Macron, who will from Saturday chair the three-day G7 summit, was on Friday due to meet Iranian Foreign Minister Mohammad Javad Zarif in Paris.

Macron admitted in comments Wednesday that there were "true disagreements" within the G7 over Iran but said he would "try to propose things" in the talks with Zarif.