The US leader was rebuffed last month when he demanded private guarantees that no strike would go ahead without White House notification, suggesting Israel no longer plans to “seek Washington’s permission”, sources said. The disclosure, made by insiders briefed on a top-secret meeting between America’s most senior defence chief and Benjamin Netanyahu, Israel’s hawkish prime minister, comes amid concerns that Iran’s continuing progress towards nuclear weapons capability means the Jewish state has all but lost hope for a diplomatic solution.

On Tuesday, UN weapons inspectors released their most damning report to date into Iran’s nuclear activities, saying for the first time that the Islamic republic appeared to be building a nuclear weapon. It was with that grave possiblity in mind that Leon Panetta, the US defence secretary, flew into Israel last month on what was ostensibly a routine trip.

Officially, his brief was restricted to the Middle East peace process, but the most important part of his mission was a private meeting with Mr Netanyahu and the defence minister, Ehud Barak. Once all but a handful of trusted staff had left the room, Mr Panetta conveyed an urgent message from Barack Obama. The president, Mr Panetta said, wanted an unshakable guarantee that Israel would not carry out a unilateral military strike against Iran’s nuclear installations without first seeking Washington’s clearance.

The two Israelis were notably evasive in their response, according to sources both in Israel and the United States.