Boris Johnson will attempt to persuade United States politicians not to quit the Iran nuclear deal amid a political firestorm at home over his remarks about a British woman jailed in the middle eastern country.

Ahead of meetings with congressional leaders in Washington DC, Mr Johnson said supporting the agreement does not mean ignoring “disruptive” Iranian behaviour such as the “unjustified detention of British nationals”.

The Foreign Secretary’s attempts at diplomacy on Iran come with him facing calls to quit over his comments about British woman Nazanin Zaghari-Ratcliffe.

Mr Johnson told a committee of MPs last week that Ms Zaghari-Ratcliffe was training journalists, rather than holidaying, in Iran at the time of her arrest last year, something her employer and family insist is incorrect.

Amid fears his comments may have led the Iranian authorities to try to extend her five-year jail sentence for supposed involvement in a coup plot, Mr Johnson on Tuesday admitted he “could have been clearer” and was “sorry” if his remarks were misconstrued.

Nazanin Zaghari-RatcliffeNazanin Zaghari-Ratcliffe is serving a five-year jail sentence in Iran (Gareth Fuller/PA)

He will now spend two days on Capitol Hill for meetings with the likes of US House of Representatives speaker Paul Ryan and majority leader Kevin McCarthy, as well as Senate majority leader Mitch McConnell.

They come after President Donald Trump decertified the Iran deal, giving Congress the power to reimpose economic sanctions on Tehran which were lifted in return for limitations to its nuclear programme.

Mr Johnson will stress the need to stick to the deal to stop Iran developing nuclear weapons.

Speaking at the start of his visit, he said: “The Iran nuclear deal makes the world safer. That’s why it is vital that the international community sticks to the deal. It is working and has, so far, resulted in Iran giving up 95% of its uranium stockpile.

“These are the points I will be making in my meetings in the United States this week.

“Supporting the nuclear deal does not mean we should not call out and take action against disruptive Iranian behaviour elsewhere, including its ballistic missile programme and the unjustified detention of British dual-nationals.

“However, it is vital that we do not conflate the issues on which we should rightly condemn Iran and a deal which is neutralising the threat of a nuclear-armed Iran.

“It took 13 years of tireless diplomacy between the UK, US, our European partners and Iran to make the world a safer place.

“Now is not the moment to put that at risk but rather it is time for the US and UK to draw on the strength of our relationship and to focus on addressing Iran’s destabilising activity in the region.”

Mr Johnson will meet Republican senator Bob Corker, who has been critical of Mr Trump, as well as the chairmen and ranking members of the Senate Foreign Relations and House Foreign Affairs Committees, senator Ben Cardin and congressmen Ed Royce and Eliot Engel.

Addressing criticism over his comments about Ms Zaghari-Ratcliffe in the Commons on Tuesday, Mr Johnson said the Government “has no doubt that she was on holiday” in Iran and that was the sole purpose of her visit.

He insisted his remarks to the committee could provide no reason for lengthening her sentence.

Before updating MPs, Mr Johnson told his Iranian counterpart Javad Zarif there was “no justifiable basis” for further legal action against the woman, who was summoned before an Iranian court on Saturday to be told she was now facing allegations of “propaganda against the state”.

Her family fear this charge could lead to a further five years’ imprisonment.