WASHINGTON — Britain will add its voice to President Obama’s in discouraging an Israeli military strike on Iran when Prime Minister David Cameron begins a three-day visit here this week, a senior British diplomat said Monday.

“The prime minister is pretty clear that he does not think military action against Iran would be helpful,” the diplomat, Peter Westmacott, Britain’s recently appointed ambassador to the United States, told reporters. “We do not regard that as the right way forward in the months to come.”

Mr. Cameron, he said, supports Mr. Obama’s vow that Iran will not be allowed to acquire nuclear weapons. And, like the president, the prime minister believes military force must be preserved as an option. But an Israeli strike, Mr. Westmacott said, could “unleash a whole variety of different consequences” and might backfire by strengthening the Iranian regime and the resolve of the Iranian people to acquire nuclear status.

“We are, all of us, concerned about what might flow from a decision to take precipitate military action,” he said.

Mr. Cameron, in his talks with Mr. Obama, is expected to press for tighter diplomatic and economic sanctions, said a senior British official in London. “There’s a lot more to be done to turn up the pressure, to turn up the dial,” said the official, who spoke on condition of anonymity to discuss the talks with the White House.

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