The nuclear deal struck with Iran in 2015 should be kept because it allows “intrusive inspections”, Boris Johnson has said.
The Foreign Secretary intervened after Israel accused Tehran of covering up its weapons programme before signing the deal three years ago.
Mr Johnson said a presentation by Israeli Prime Minister Benjamin Netanyahu “underlines the importance” of keeping the deal’s constraints on Iran in place.
At a time of growing uncertainty, Donald Trump has set a 12 May deadline to decide whether to pull the US out of the accord.
Mr Netanyahu said “half a ton” of secret Iranian documents obtained by his government showed Iran was “brazenly lying”.
Later, he told CNN he was not seeking war with Iran.
After signing the deal in 2015, Iran was obliged to limit its nuclear energy programme, amid fears the country could use the technology to make a nuclear weapon.
Economic sanctions were lifted in return. Iran has denied ever seeking nuclear weapons.
US Secretary of State Mike Pompeo said he had “personally reviewed many of the Iranian files”.
They showed that Iran “had a secret nuclear weapons program for years”, he added.
A senior Israeli official said Mr Netanyahu had told Mr Trump about the documents on 5 March, and that the US President agreed that Israel would publish the information before 12 May.
In addition to the UK, the EU, France and the UN’s nuclear watchdog, the International Atomic Energy Agency (IAEA), have also cast doubt on the significance of Mr Netanyahu’s presentation.
Explaining his support for the 2015 agreement, Mr Johnson said it was “not based on trust about Iran’s intentions; rather it is based on tough verification, including measures that allow inspectors from the IAEA unprecedented access to Iran’s nuclear programme”.
The IAEA said it believed that Iran had a “coordinated” nuclear weapons programme before 2003, but had found “no credible indications” of such work after 2009.
Mr Johnson added: “The fact that Iran conducted sensitive research in secret until 2003 shows why we need the intrusive inspections allowed by the Iran nuclear deal today.
“The verification provisions in the Iran nuclear deal would make it harder for Iran to restart any such research.
“That is another good reason for keeping the deal while building on it in order to take account of the legitimate concerns of the US and our other allies.”
The European Union’s foreign affairs chief, Federica Mogherini, said she was unsure whether Mr Netanyahu’s presentation had changed the narrative.
“What I have seen from the first reports is that Prime Minister Netanyahu has not put into question Iran’s compliance,” she said.
And a spokesman for France’s foreign ministry said the “pertinence of the deal is reinforced by the details presented by Israel”.
Iran described Mr Netanyahu as an “infamous liar”.
Israel’s allegations were “worn-out, useless and shameful”, said foreign ministry spokesman Bahram Ghasemi.