The world’s big powers will on Saturday conduct their highest-level talks with Iran for two years, in a new attempt to persuade Tehran to rein in its nuclear programme in return for political and economic rewards.

In what diplomats say is a last-ditch initiative before the European Union imposes more sanctions on Tehran, Javier Solana, the EU’s foreign policy chief, will lead senior diplomats from the UK, France, Germany, Russia and China in discussions with Iranian leaders.