For 70 years, even as its colonies have revolted across Africa and Asia and its global empire has collapsed, Britain has held on to a vital piece of power: a permanent veto-wielding seat on the United Nations Security Council.

Britain’s exit from the European Union makes that seat more important than ever, particularly for the United States. Britain has been America’s most reliable ally on the council, and an important counterweight to China and Russia, also permanent members.

“The U.K. will hang on to its permanent seat for dear life, just as Moscow did after the collapse of the U.S.S.R.,” said Richard Gowan, a Briton based in Brooklyn, who studies the United Nations as a nonresident fellow at the Center on International Cooperation at New York University.

“It is one of the few tokens of British political prestige that ‘Brexit’ doesn’t affect, at least in formal terms,” Mr. Gowan said.

Matthew Rycroft, Britain’s ambassador to the United Nations, reinforced that idea when he said in a brief interview that Britain’s role on the Council would remain unchanged.

“The U.K. is proud to be a permanent member of the Security Council,” he said. “And we have always played that role responsibly and actively and that will continue.”

The Council reflects a world that no longer exists. The five permanent seats were claimed by the victors of World War II: Britain, China, France, Russia and the United States.

(The Council’s 10 rotating nonpermanent members have minimal powers.)

France will now become the sole permanent member of the Council that is also a member of the European Union, giving that country new impetus to justify its presence.

But other countries, which have long complained about the composition of the Council’s permanent membership, may see Britain’s referendum as a chance to make a push for their own veto-wielding seats.

The move could motivate Germany to seek a permanent seat, or encourage those who want to see a rotating permanent seat for European Union members.

In reality, that prospect is slim, diplomats say. Rejiggering the Council is mired in tense global rivalries that have little to do with Europe, and lots to do with the rise of Asia. Japan also wants a permanent seat, which China eschews. And so Council “reform,” as its called, has been stalled for years.

And what about Britain’s role going forward?

Britain has been a steadfast American ally on thorny global conflicts, including conflicts in Syria, Ukraine and Yemen. It has also been a vital European partner, most recently in securing the Council’s blessings to authorize European naval ships to interdict migrant and arms smugglers moving in and out of Libya.

And as Daniela Schwarzer, the Berlin-based director of the Europe program at the German Marshall Fund of the United States, wrote in a blog post Friday morning: “It is still in the interest of continental Europe to keep close relationships with the U.K., and to help avoid substantive weakening of a country that used to be known for its healthy pragmatism.”