Britain has scaled back a high-level meeting on ending the war in Ukraine, according to European officials, after Secretary of State Marco Rubio decided to pull out on Tuesday. That prompted the British foreign secretary, David Lammy, to decide that he, too, would not attend, though he will meet separately with Ukraine’s foreign minister in London.

Lower-level diplomats from Britain, France, Germany, Ukraine, and the United States will still meet on Wednesday to hold technical talks, according to the officials from Britain and France. But the sudden downgrading of what was meant to be a significant diplomatic gathering raises questions about efforts to negotiate a cease-fire in the war between Russia and Ukraine.

The diplomats are expected to hear details of a proposed peace plan from the Trump administration. Among its provisions: Ukraine must accept the legal annexation of Crimea by Russia, and it cannot become a member of NATO. President Volodymyr Zelensky of Ukraine on Tuesday rebuffed demands that his country accept the loss of Crimea, saying that doing so would go against its constitution.

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