Iran must make a "big change" in its strategic thinking, David Cameron warned today, as leading international powers agreed to resume talks with the Tehran regime over its nuclear programme.
The Prime Minister said that while economic sanctions should be given more time to run, he made clear military action remained an option unless the Iranians gave up any ambitions to develop a nuclear weapon.
"Nothing is off the table," Mr Cameron told the Commons Liaison Committee.
"It is difficult to say that because no one wants to see conflict in any way. But I think it's very important that world sends a message to Iran that a nuclear-armed future is not something that we want to see.
"If the sanctions don't work there will come a moment of a very difficult decision."
Earlier, the Government's national security adviser Sir Kim Darroch, and other "experts" briefed the weekly Cabinet meeting on the latest situation.
Mr Cameron's comments came as the so-called E3+3 group - Britain, France and Germany plus the United States, Russia and China - accepted an offer from the Iranian nuclear negotiator Saeed Jalili to re-open discussions after the last round ended in failure in January 2011.
The international powers are demanding Tehran gives up its nuclear enrichment work - a key step in developing a nuclear bomb.
Mr Cameron said Iran's decision to return to the negotiating table was evidence that the latest sanctions - including an EU oil embargo - were having an effect.
And with Israel showing increasing signs of impatience, he echoed US president Barack Obama in appealing for more time to find a diplomatic solution to the crisis.
"Today, we think that military action against Iran and Israel would not be the right approach. We have said that both publicly and privately to the Israelis. We think this track of sanctions and pressure has further to run," he said.
Mr Cameron insisted no decisions had been made about any British involvement in any future US military action against Iran. However he pointed out that the UK had military assets in the region, including Royal Navy minesweepers.
"We haven't made decisions about any military action but clearly we would want to consider not least how best to protect our own interests and our own people," he said.
He stressed that a nuclear-armed Iran would not just be a threat to Israel and other countries in the region.
"I don't believe that an Iranian nuclear weapon is just a threat to Israel," he said.
"It is also clearly very dangerous for the region because it would trigger a nuclear arms race but also its a danger more broadly, not least because there are signs that the Iranians want to have some sort of intercontinental missile capability.
"So we have to be clear this is potentially a threat much more widely."