David Cameron has issued a renewed appeal to Israel not to launch military action against Iran over its nuclear programme.
The Prime Minister insisted international sanctions had created "unprecedented pressure" on the regime and and should be given a chance to work.
However, in a speech to be delivered to the United Jewish Israel Appeal in London, he also warned the Iranians that if they made the "wrong choice" and did not give up work on uranium enrichment, then "nothing is off the table".
His intervention came as European Union foreign ministers meeting in Luxembourg agreed a further tightening of existing financial, trade and energy sanctions.
In his speech, Mr Cameron is expected to reject claims the sanctions strategy is "fatally flawed" and that the international community either has to accept the inevitability of a nuclear-armed Iran or take military action to halt it.
He warns that if Israeli Prime Minister Benjamin Netanyahu orders a military strike now, it would simply play into the hands of the regime.
"I have said to Prime Minister Netanyahu that now is not the time for Israel to resort to military action," he says.
"Beyond the unpredictable dangers inherent in any conflict, the other reason is this: at the very moment when the regime faces unprecedented pressure and the people are on the streets; and when Iran's only real ally in Syria is losing his grip on power, a foreign military strike is exactly the chance the regime would look for to unite his people against a foreign enemy. We shouldn't give them that chance. We need the courage to give these sanctions time to work."
Mr Cameron says that the "relentless sanctions" are now having an impact that was not expected even a year ago.
"Most significantly, there are signs that the Iranian people are beginning to question the regime's strategy with even pro-regime groups protesting at the actions of the government," Mr Cameron says. "The Iranian regime is under unprecedented pressure and faces an acute dilemma. They are leading their people to global isolation and an economic collapse. And they know it."