Johnson said that the 2015 agreement "has helped to avoid a possible catastrophe".
The statement was released on Saturday at the Second Session of the Preparatory Committee for the 2020 Review Conference of the Parties to the Treaty on the Non-Proliferation of Nuclear Weapons.
Since the accord between Iran and the six world powers was signed in Vienna in July 2015, the International Atomic Energy Agency (IAEA) has repeatedly confirmed Iran's compliance with its commitments.
President Donald Trump could start a war if he chooses to fully withdraw from the Iran deal later this month, French President Emmanuel Macron, who recently visited the White House to discuss the matter, warned over the weekend. Trump has called it the worst deal in USA history, once suggesting it was so bad that "there has to be something else going on". He also takes issue with limits on which military site inspectors can see and the deal's failure to address Iran's ballistic missiles program and terrorist support.
"In effect, I don't know what Trump will decide because he hasn't told me", the senior Israeli official said on condition of anonymity, making clear Israel had not yet been informed of Trump's decision.
European aeroplane manufacturer ATR struck a US$536-million deal with IranAir for at least 20 aircraft a year ago.
Britain, France and Germany have been working behind the scenes for weeks in an effort to preserve the deal, which was orchestrated under the Obama administration, and includes Russian Federation and China as signatories.
Restrictions on Iran's nuclear activities begin expiring in 2025.
While observers believe that Trump will stay in the deal after the May 12 deadline, the President has not given anything away, even in the face of intense lobbying by US allies, including the United Kingdom, France and Germany.
It remains unclear what would happen if the USA withdraws from the deal but the other countries remain committed.
President Hassan Rouhani made the comments Sunday in the city of Sabzevar while on a tour of Iran's Razavi Khorasan province.
World powers that were party to the agreement, including Britain and France, said those arguments only strengthened the reasoning for the deal, which has safeguards in place created to keep Iran from pursuing nuclear weapons.
He stated: "Either fix the deal's disastrous flaws, or the United States will withdraw".
Q: What would the impact be if the president declined to recertify the deal? At least 10 per cent of its aircraft components are of American origin, meaning it requires permission from the US Treasury for its sales to Iran.
During Macron's three-day visit to Washington in April, he urged Trump to uphold the 2015 deal between Iran, the US and other world leaders that had been brokered by Trump's predecessor, President Barack Obama.