Ukraine’s president Volodymyr Zelensky became the first foreign leader to attend Britain’s cabinet meeting in person since former U.S. president Bill Clinton in 1997, in a public display of Britain’s staunch support for Ukraine as doubts grow about future U.S. military aid if Donald J. Trump wins a second presidential term.

Mr. Zelensky briefed the British government’s top ministers on Friday on his country’s military conflict with Russia, while discussing moves to prevent oil tankers from breaching international sanctions against Moscow.

The Ukrainian president, who was greeted with a standing ovation, was the first foreign leader to be invited into Downing Street by Keir Starmer, the new British prime minister, following his general election victory earlier this month. The two leaders also discussed a new defense export support treaty designed to boost production of military hardware and weaponry in both countries.

Mr. Zelensky’s visit to Downing Street followed his attendance on Thursday at a summit of more than 45 European leaders at Blenheim Palace, near Oxford, where Ukraine was high on the agenda, and where he won renewed pledges of support for his battle against the forces of Russia’s president, Vladimir V. Putin.

“Ukraine is, and always will be, at the heart of this government’s agenda and so it is only fitting that President Zelensky will make a historic address to my Cabinet,” Mr. Starmer said in a statement issued by his office ahead of the meeting.

Later on Friday, Downing Street said that Mr. Starmer had welcomed Mr. Zelensky by saying that the meeting was a piece of history and the first time this century that a foreign leader has addressed the gathering in person.