Iran’s president has warned Donald Trump that he would be making a “historic” mistake if the US were to withdraw from a global nuclear deal with Tehran, as the UK foreign secretary flew to Washington to help save the deal.

Hassan Rouhani said in a televised speech on Sunday that Iran had “plans to resist any decision by Mr Trump on the nuclear accord”. He said if the US pulled out of the deal by the May 12 deadline, it would “soon realise that this decision will become a historic regret for them”.

Boris Johnson, Britain’s foreign minister, is the third senior representative of the so-called E3 — the European powers of Britain, France and Germany — to urge the Trump administration to stick to the deal, which global powers agreed in 2015 to ease sanctions on Tehran in exchange for Iran limiting its nuclear ambitions.