Tehran is “pressing the gas pedal” on the refinement of uranium and has its largest stockpile to date, the UN's top nuclear weapons inspector said.

Rafael Grossi, director general of the International Atomic Energy Agency, said Iran has about 200kg of uranium enriched to up to 60 per cent purity. That is close to the roughly 90 per cent required for use in a weapon, and in principle enough for about five nuclear bombs.

“They are pressing the gas pedal. It is for you to interpret and give your analysis of that, but this is the fact – we have seen it,” said Mr Grossi at the World Economic Forum in Davos, following a visit to Iran in November.