Britain is considering sending tanks to Ukraine, in what would be a first.

Britain is weighing whether to send a small number of tanks to Ukraine, potentially reversing the West’s nearly yearlong resistance to deploying some of its mightiest firepower against Russia.

No decision has yet been made on whether the Challenger II tanks — reportedly as few as 10 — will be donated to Ukraine. But such a move is being considered,Mr. Sunak’s official spokesman told reporters at a Downing Street briefing on Tuesday, noting that Rishi Sunak, Britain’s prime minister, had spoken with President Volodymyr Zelensky of Ukraine a week earlier “about what would be most effective in helping Ukraine.”

Kyiv has demanded Western tanks almost since the start of the war to supplement stocks of Soviet-era or Russian-made tanks that ’s stocks of

The Challenger II would be the first Western-made main battle tank to be sent to Ukraine since Russia invaded last February. Defense officials in the United States and Europe have long worried that sending tanks would signal more direct involvement in the fighting, and could prompt President Vladimir V. Putin of Russia into escalating. The internal British discussions were reported earlier by SkyNews.

The move follows pledges by other Western powers to give Kyiv heavy armored fighting vehicles amid an ongoing counteroffensive in eastern Ukraine. The country’s foreign minister, Dmytro Kuleba, over the weekend welcomed such pledges as a sign that “the time of weapons taboo has passed.”

Last week, France said it would deliver an unspecified but limited number of French AMX-10 reconnaissance vehicles. That announcement was quickly followed by a decision by the United States to send Ukraine 50 M2 Bradley Fighting Vehicles, and by Germany, which pledged 40 Marder Infantry Fighting Vehicles.

Sending Challenger IIs could ratchet up pressure on Germany to commit to sending its Leopard II tanks to Ukraine, as Kyiv wants. Chancellor Olaf Scholz has maintained that Berlin would not be the first NATO ally to send such equipment into the war.

On Tuesday, Ukraine’s foreign minister, Dmytro Kuleba, met with Germany’s foreign minister, Annalena Baerbock, as she visited eastern Kharkiv, and urged Germany to send Leopards. “The longer it takes to make the decision, the more people will die,” he said at their joint news conference, according to news reports. “The sooner this decision is made, the sooner this war will end with Ukraine’s victory and there will be no more war in Europe.”

Ben Barry, a land warfare expert at the International Institute for Strategic Studies in London, said that the Challenger II was similar to the Leopard II and to the American-made M1 Abrams tank.

But, he said, Britain has only about 227 Challenger II tanks on hand, and ammunition and spare parts are limited.

By comparison, there are several thousand German-made Leopards used by national militaries across Europe, Mr. Barry said, although Berlin must give its approval before they can be donated to Ukraine. Germany has strict guidelines on exports and re-exports of its sensitive military equipment transfers to countries engaged in combat.

“It shouldn’t be too difficult for Europe to find most of the additional 300 Western tanks that Ukraine is asking for,” Mr. Barry said on Tuesday.

“Ukraine has said that it wants to mount significant offensive operations this coming year, in order to push the Russians back — ideally to push them out,” Mr. Barry said. “And it’s said to do that, it needs extra Western armor.”