British Foreign Secretary David Lammy says he does not accept Iran's claims that the country is enriching uranium for academic purposes.
Representatives from the United Kingdom, Germany, and France held talks with Iran last week to try to break the deadlock over the country's nuclear programme.
Tehran maintains it is open to diplomacy, though it recently suspended cooperation with the International Atomic Energy Agency (IAEA).
A central concern for western powers was highlighted when the IAEA reported in May that Iran's stockpile of uranium enriched to 60% - just below weapons-grade level - had grown to more than 400kg [$ conversion missing].
In a wide-ranging interview with The Guardian newspaper, Lammy said: "Its leaders cannot explain to me - and I've had many conversations with them - why they need 60% enriched uranium.
"If I went to Sellafield or Urenco in Cheshire, they haven't got anything more than 6%. The Iranians claim it's for academic use, but I don't accept that."
Lammy warned that Iran developing nuclear weapons could lead to an escalation of tensions in the Middle East.
Israel and the United States carried out strikes on Iranian nuclear facilities in June.
"Many of your readers will have watched Oppenheimer and seen the fallout of (the US building an atomic bomb)," he said.
"So it's what (a nuclear Iran) might mean in terms of other countries in the neighbourhood who would desire one, too. And we would be very suddenly handing over to our children and grandchildren a world that had many more nuclear weapons in it than it has today."
The foreign secretary said he had heard Israeli arguments in favour of regime change in Tehran, but did not believe that was behind the US decision to strike.
The Tottenham MP added any decision to topple the government was one for the Iranian people, with his focus "on what the UK can do to stop Iran becoming a nuclear power."
Last month, Lammy suggested that Britain, France, and Germany could "snap back" on sanctions against Iran unless the country gets "serious" about stepping back from its nuclear ambitions.
He told the Commons Foreign Affairs Committee: "Iran face even more pressure in the coming weeks because the E3 can snap back on our sanctions, and it's not just our sanctions, it's actually a UN mechanism that would impose dramatic sanctions on Iran across nearly every single front in its economy.
"So they have a choice to make. It's a choice for them to make.
"I'm very clear about the choice they should make, but I'm also clear that the UK has a decision to make that could lead to far greater pain for the Iranian regime unless they get serious about the international desire to see them step back from their nuclear ambitions at this time."
