A Foreign Office spokesperson said: 'We have received reports that the Iranians have decided to restart their Uranium Conversion facility at Isfahan, where activities have been suspended since November last year in accordance with successive IAEA board resolutions and the terms of the Paris Agreement between Iran and the E3 (Britain, France and Germany).

'This would be an unnecessary and damaging step by Iran. The E3 Foreign Ministers and the EU High Representative have just written to Dr Rouhani - Iran's chief negotiator on nuclear activities - confirming that full and detailed proposals would be given to Iran in a week's time, in accordance with the decisions at the E3/Iran ministerial meeting in Geneva in May.

'We are seeking clarification of Iran's intentions. We urge them not to take any unilateral step which would contravene the Paris agreement as that would make it very difficult to continue with the E3/Iran negotiations.

'Should the Iranians persist, we will as a first step consult urgently with our partners on the board of the IAEA.'