Russia on Friday summoned the British charge d'affaires in Moscow over what it called an "escalation" of arms supplies to Ukraine, accusing the UK of having a "confrontational stance".
Russia considers Britain to be one of the most hostile Western countries, with relations deteriorating long before the Ukraine war.
Britain is Ukraine's main allies, and new prime minister Andy Burnham went to Kyiv as his first trip overseas.
Moscow's foreign ministry said the UK envoy Danae Dholakia was given a "strong protest against London's continued escalation of supplies of drones and other weapons" to Kyiv.
She was told that Britain's "highly confrontational stance on the Ukrainian issue is clearly aimed at prolonging and escalating" the war, the statement added.
Dholakia was told that "the Russian side reserves the right to take the necessary counter-measures in the exercise of its right to self-defence".
Moscow also said that "any British military facilities in Ukraine used to launch strikes against Russian territor are legitimate targets".
In response, a British Foreign Office spokesperson said: "This is yet another attempt to deflect attention from Russia's barbaric full scale invasion of Ukraine and discredit the UK on the world stage.
"We are committed to providing the equipment Ukraine needs to defend itself and will not let up on our support to secure a just and lasting peace."
Britain this month announced it would commit 100 million pounds ($135 million) to boost Ukraine's air-defence capabilities this winter, including with Patriot missiles.
It also said it would supply "air-defence interceptors, large-calibre artillery rounds and drones" to a NATO programme supporting Kyiv.
Russia's war against Ukraine has dragged on for more than four-and-a-half years.