The E3 letter follows months of diplomatic stalemate, particularly after a 12-day war between Iran and Israel in June.

The UK, France and Germany have told the UN they are ready to reimpose sanctions on Iran over its nuclear programme if it fails to resume talks by the end of August. The three countries, known as the E3, said they were prepared to trigger a "snapback" mechanism - meaning previous sanctions would be reinstated - unless Iran resumes negotiations.

Britain, Germany and France have told UN officials that snapback sanctions are on the table if Iran does not sit down to negotiate over its nuclear weaponry.