KYIV — The British government said Saturday that the Kremlin was developing plans to install a pro-Russian leader in Ukraine — and had already chosen a potential candidate — as President Vladimir V. Putin weighs whether to order the Russian forces amassed on Ukraine’s border to attack.

The highly unusual public communiqué by the United Kingdom’s Foreign and Commonwealth Office, issued late at night in London, comes at a moment of high-stakes diplomacy between the Kremlin and the West. Russia has deployed more than 100,000 Russian troops on Ukraine’s borders that could, according to American officials, attack at any moment.

“The information being released today shines a light on the extent of Russian activity designed to subvert Ukraine, and is an insight into Kremlin thinking,” Liz Truss, Britain’s foreign secretary, said in a statement. “Russia must de-escalate, end its campaigns of aggression and disinformation, and pursue a path of diplomacy.”

The communiqué provided few details about how Russia might go about imposing a new government on Ukraine, and did not say whether such plans were contingent on an invasion by Russian troops. British officials familiar with the situation, who spoke on condition of anonymity, said the intent was both to head off the activation of such plans as well as to put Mr. Putin on notice that this plot had been exposed.

In Washington, officials said they believe the British intelligence is correct. Two officials said it had been collected by British intelligence services. Within the informal intelligence alliance known as “Five Eyes,” Britain has primary responsibility for intercepting Russian communications, which is why it played a major role in exposing Russian interference in the 2016 elections.

Emily J. Horne, the spokeswoman for the U.S. National Security Council, said in statement that “this kind of plotting is deeply concerning. The Ukrainian people have the sovereign right to determine their own future, and we stand with our democratically-elected partners in Ukraine.”

But the Russian foreign ministry denied the British accusation.

“The spread of disinformation by the British foreign ministry is one more piece of evidence that NATO countries, led by the Anglo Saxons, are escalating tensions around Ukraine,” it said in a statement. “We call on the British foreign ministry to stop its prevocational activities.”

Ukraine is in a state of high anxiety. In recent weeks several reports have emerged about plots and schemes aimed at destabilizing the government and tipping the country into war.

This month, the United States accused the Kremlin of sending saboteurs into eastern Ukraine to create a provocation that could serve as a pretext for invasion. Ukraine’s military intelligence agency said Russia has recently sent hundreds of mercenaries into two rebel eastern Ukrainian regions, and last November President Volodymyr Zelensky said Ukrainian intelligence had uncovered a Russian-backed coup plot involving a prominent Ukrainian oligarch.

At a security conference in Kyiv on Saturday, participants, most of them high ranking members of Ukraine’s political opposition, spoke darkly of fifth columnists and enemy collaborators.

“We are not just talking about large-scale aggression by Russia,” said Pavlo Klimkin, a former foreign minister. “We are talking about the wish of Russian officials, including Putin, to destroy Ukraine as such.”

The British communiqué provided no evidence to back up its assertion that Russia was plotting to overthrow the Ukrainian government. The communiqué also named four other Ukrainians, accusing them of maintaining ties to the Russian intelligence services, including Russian intelligence officers involved in the planning for an attack on Ukraine.

Of the five Ukrainians named, four fled Ukraine for Russia in 2014 following a popular uprising that ousted the Russian-backed government in Kyiv and touched off the separatist war in Ukraine’s east that continues today.

According to the British assessment, Russian planners were considering installing a former member of the Ukrainian Parliament named Yevgeniy Murayev as leader of a pro-Kremlin puppet government in Kyiv. Once a member of the Russian-backed Party of Regions, Mr. Murayev is now head of a political party called Nashi, part of a constellation of opposition parties opposed to Ukraine’s pro-Western parties. Last September, a massive banner with his photograph was hung on the facade of Federation of Trade Unions building on Kyiv’s Independence Square with the slogan “This is our Land.”

In a recent Facebook post he accused the current government in Kyiv of selling out to the United States, which he said was whipping up war hysteria to reap financial rewards from the sale of weapons.

“The hawks are looking forward to a feast,” he wrote.

It is not clear from the British statement whether Russia had informed Mr. Murayev that he was being considered as a possible future leader of Ukraine. But after a journalist with a British newspaper tipped him off about the revelations, he posted a photo of himself to Facebook posing as James Bond with the comment, “Details tomorrow.”

Russian spies maintain extensive networks of agents in Ukraine and contacts between Ukrainian officials and intelligence officers are not uncommon, according to Ukrainian and Western security officials

All four of the other Ukrainians named in the communiqué once held senior positions in the Ukrainian government and worked in proximity to Paul Manafort, former President Donald J. Trump’s campaign manager, when he worked as a political adviser to Ukraine’s former Russian-backed president, Viktor F. Yanukovych. After Mr. Yanukovych’s government fell in 2014, they fled to Russia.

One of those named, Vladimir Sivkovich, was among four Ukrainians targeted last week with sanctions by the United States Treasury Department for their ties to Russian efforts to destabilize Ukraine.

If the British assessment is accurate, it would not be the first time the Kremlin tried to install a pro-Russian leader or interfere in Ukraine’s government. In 2004, Russian efforts to fraudulently sway a presidential election set off what became known as the Orange Revolution, which forced a redo election that led to the defeat of Mr. Yanukovych, who was the Kremlin’s favored candidate.

In 2013, when the Kremlin pressured Mr. Yanukovych, who eventually was elected president, to back out of a trade pact with the European Union, Ukrainians again poured into the streets. Mr. Yanukovych was eventually driven from power, prompting Mr. Putin to order the annexation of the Crimean Peninsula and instigate a separatist war in eastern Ukraine.

Russian officials have repeatedly denied any intention of launching an attack against Ukraine, dismissing such accusations as “hysteria” and claiming without providing evidence that it is the government in Kyiv that is seeking to escalate tensions. Even so the buildup of Russian troops on the border has continued. At least 127,000 soldiers now surround Ukraine to the north, east and west, Ukraine’s military intelligence service says, with additional troops from Russia’s Eastern Military District now pouring into neighboring Belarus.

The standoff is redolent of an old-fashioned Cold War showdown between Moscow and the West, with both sides trading accusations of war mongering and jockeying for geopolitical advantage. Though the confrontational tone was muted when Secretary of State Antony J. Blinken met his Russian counterpart for the latest round of talks in Geneva on Friday, there is as yet no end in sight.

Britain’s unusual disclosure comes at a time when it is trying to assert itself in the crisis on military and diplomatic fronts. It has delivered shipments of antitank weapons to the Ukrainian military, dispatched its senior ministers to NATO countries under threat from Russia and begun to engage directly with Russia.

Britain’s defense secretary, Ben Wallace, accepted an invitation from his Russian counterpart, Sergei K. Shoigu, to meet in Moscow, while the foreign secretary, Liz Truss, may meet with Russia’s foreign minister, Sergey V. Lavrov.

The disclosure also comes amid a swirling political scandal over Downing Street garden parties in 2020 that violated lockdown restrictions, which has mushroomed to such a degree that it threatens Prime Minister Boris Johnson’s hold on power.

Critics have suggested that Mr. Johnson may try to exploit the tensions with Russia — and Britain’s more assertive diplomatic and military role — as a way to deflect attention from his political woes.

Kenneth P. Vogel contributed reporting from Washington and Maria Varenikova reported from Kyiv.