When Prime Minister Keir Starmer swept out the Conservative government in July, he promised a new era for British politics and a reset of its relations with the European Union. One thing hasn’t changed: Britain’s bitter dealings with an aggressive Russia. If anything, the bad blood has gotten worse.

Tensions between London and Moscow spiked this week over signs that the United States, prodded by Britain, is moving toward allowing Ukraine to use Western-supplied missiles to strike military targets deep inside Russian territory.

As Mr. Starmer arrived in Washington on Friday for potentially decisive talks with President Biden, he and President Vladimir V. Putin of Russia traded harsh words, while Russia said it had expelled six British diplomats.

The missiles are becoming the first major foreign policy test of Mr. Starmer’s premiership — one with security implications for all of Europe. Britain’s diplomatic offensive has put it at the vanguard of a broader European campaign for support of Ukraine, at a moment of deep political uncertainty in the United States, which could limit a future American role in resisting an advancing Russia.

Mr. Starmer and Mr. Biden are expected to discuss the terms under which Ukraine would get a green light to use Britain’s “Storm Shadow” long-range missiles inside Russia. Getting Mr. Biden to sign off on such strikes is critical, analysts said, for symbolic and operational reasons: the missiles use satellite data and other technology supplied by the United States.

“We want to move with the Americans on this,” said Peter Ricketts, a former British national security adviser. “We’re now in a period where Biden isn’t running for office, so he can focus completely on his legacy. Starmer wants to play into that to encourage him to go as far as he can on Ukraine.”