A final push to settle the confrontation over Iran's nuclear ambitions was in the balance last night as John Kerry postponed his return to America to press on with talks in Switzerland.
The US secretary of state spent a fourth day in meetings in Lausanne with Mohammad Javad Zarif, the Iranian foreign minister, as the clock ticked towards tomorrow's deadline for a agreement.
Sensing the possibility of a historic deal after more than a decade of diplomacy, the world's most powerful foreign ministers converged on the town. They included Sergei Lavrov, the Russian foreign minister, and Wang Yi, his Chinese counterpart.
Philip Hammond, the British Foreign Secretary, said: "A comprehensive nuclear deal with Iran is in all our interests. Both sides now need to work intensively to bridge the remaining differences. That will mean some tough choices if we are to reach what would be a historic deal."
The talks with Iran are being conducted by the UN Security Council's five permanent members - China, France, Russia, Britain and the US - plus Germany.
Iran and the "P5+1" are believed to remain at odds over two key issues: the pace in which sanctions would be lifted under a deal and the right of Iran's scientists to continue developing advanced centrifuges.
But the negotiators are close to resolving perhaps the most contentious question of all. America wants to ensure that Iran's scientists would need at least a year to produce enough weapons-grade uranium for one nuclear bomb.
Iran is believed to have provisionally agreed to meet this demand by sacrificing at least a third of its 9,900 operational centrifuges and exporting almost all of its 7.9 tonnes of low-enriched uranium.
In return, Iran wants all sanctions to be lifted as soon as a deal is signed. Mr Kerry, for his part, will agree only to a gradual easing of the economic pressure, linked to the steps that Iran would take.
"We're getting to the final hours where people have got to ask what their red lines are - and if they're really red lines," said a diplomat in Lausanne.
The prospect of a deal is sufficiently realistic to alarm Israel's government. Benjamin Netanyahu, the prime minister, believes that allowing Iran to continue enriching uranium on any scale would amount to a capitulation.
He told his cabinet yesterday: "This deal, as it appears to be emerging, bears out all of our fears - and even more than that."
Comparing the talks to the capture of large areas of Yemen by Iran-backed rebels, Mr Netanyahu said: "The Iran-Lausanne-Yemen axis is very dangerous to humanity and must be stopped."
Israeli diplomats feel that the US administration no longer heeds their concerns - and that Britain's position is identical to Washington's. They have focused their efforts on the US Congress and on France, which has adopted the most hawkish stance of the Western powers. (© Daily Telegraph, London)