The UK, France and Germany are preparing on Thursday to trigger a UN process to reimpose sanctions on Iran over its nuclear programme, European officials have said, in a move that would heighten tensions between the west and Tehran.

The decision comes after foreign ministers of the three nations, known as the E3, told Iranian officials at a meeting in Istanbul last month that they were willing to extend a UN sanctions deadline if the Islamic republic restarted talks with the US and resumed co-operation with the International Atomic Energy Agency before the end of August.

Triggering the “snapback” process — a mechanism contained in the 2015 nuclear accord that Tehran signed with world powers — on Thursday would lead to the reimposition of UN sanctions after a 30-day countdown, unless there is a significant diplomatic breakthrough.