Ninety minutes after resigning as British prime minister on Thursday, Boris Johnson called Ukrainian President Volodymyr Zelenskyy. He told the Ukrainian leader his people had the U.K.'s unwavering support in its fight against Russia, and said Britain would continue to supply vital defensive aid for as long as needed. "You're a hero, Volodymyr," he said, according to an aide who listened to the call. "In this country, everybody loves you."

In the months since Russia invaded Ukraine, Britain has become an important go-between for Zelenskyy, officials in Britain and the United States said. Part broker, part delivery service, supporting Ukraine has been a crucial part of Johnson's premiership. Ukrainian Foreign Minister Dmytro Kuleba described him as a "true friend of Ukraine" in comments sent to Reuters by his ministry.

Whoever runs Britain in coming months will face important decisions about how to pursue the policy. Johnson told Zelenskyy he still had "a few weeks" to keep the support going, according to the aide. But Britain's leadership will be at a point of transition as Russia is steadily gaining ground in what Moscow calls a "special operation."