Labour has accused David Cameron of committing a gaffe by mistakenly claiming Iran has a nuclear weapon.
Asked why he was backing Turkey to join the EU, he said it could help solve the world's problems, "like the Middle East peace process, like the fact that Iran has got a nuclear weapon".
A No 10 source said the PM "misspoke", later adding he had been talking about Iran's pursuit of nuclear weapons.
But Labour said he was becoming a "foreign policy klutz".
Shadow Europe Minister Chris Bryant said: "This is less of a hiccup, more of a dangerous habit.
"Considering Iran's nuclear ambitions constitute one of the most important foreign policy challenges facing us all, it is not just downright embarrassing that the prime minister has made this basic mistake, it's dangerous."
He said Mr Cameron had been forced to "explain away another foreign policy gaffe" - a reference to the diplomatic rows that erupted over his recent comments about Pakistan.
Iran has been the focus of concern among the international community over its uranium enrichment programme, with the US military warning in April it could produce enough material for a nuclear bomb in one year.
One of Mr Cameron's aides said the prime minister "misspoke" when he said "Iran has a nuclear weapon".
But she later told the BBC: "If you watch back the prime minister's words, it is clear he is talking about Iran's pursuit of a nuclear weapon."
The prime minister made the comment at one of his "PM Direct" public meetings at Hove, East Sussex.
Immediately beforehand, he had been berated by a member of the public about an earlier gaffe, when he described Britain as the US's "junior partner" against the Nazis in 1940.
Retired telephonist Kathy Finn, 75, accused him of "denigrating" his own country, asking him: "Who do you consider was the senior partner in the Battle of Britain when we were fighting alone in the first two and a half years of the war?"
A chastened Mr Cameron replied: "There was no senior partner. We were on our own in 1940... what I meant to say was that I was referring to the 1940s, not 1940.
"You are absolutely right and I was absolutely wrong."
It comes ahead of what is expected to be a tense meeting between Mr Cameron and Pakistani President Asif Ali Zardari, following remarks last week by the prime minister while he was in India.
He provoked anger in Islamabad when he said that elements in Pakistan should not be allowed to "promote the export of terror whether to India, whether to Afghanistan or to anywhere else in the world".
Mr Cameron has continued to stand by his remarks, insisting they were a "clear and frank" response to a similarly frank question.
Mr Zardari has said he will challenge Mr Cameron personally over the remarks.
The two men will meet for an informal private dinner at Chequers ahead of formal talks at the country retreat on Friday.
Mr Zardari's visit is being seen as an opportunity to "reinforce the strong links" between the UK and Pakistan, Downing Street said.