LONDON — Few world leaders have looked weaker lately than Theresa May, the British prime minister. Yet in Parliament on Wednesday, she vowed to stand tough in the escalating confrontation with Russia over the use of a nerve agent to poison one of its former spies on British soil.
In language reminiscent of the Cold War, Mrs. May — until recently, accused at home of not being hard enough on Moscow — expelled 23 Russians she said were spies, promised a crackdown on corrupt Russians and the money they funnel into Britain, and called off high-level contacts between the two governments.
Suddenly, she is the most forceful Western leader in denouncing the government of President Vladimir V. Putin, which she portrayed this week as a malevolent and lawless force.
Her decision, which she described as the biggest expulsion of Russian diplomats in more than 30 years, makes for a particularly sharp contrast with President Trump, who has been notably reluctant to criticize Mr. Putin and is dogged by accusations that the Kremlin tried to help him in the 2016 election.
But it was not clear how strongly allies would rally to her side, and experts said that behind Mrs. May’s tough talk lay relatively mild measures, the headline-grabbing expulsion aside. That, in turn, reflects Britain’s weakened position in the world, as well as Russia’s continuing success in sowing discord and division.
Experts have described a number of tougher measures Britain could also take, like seizing any assets of questionable provenance belonging to Russians who have invested and settled in the country, changing laws that made it possible to hide the true ownership of assets, and calling on the international community to tighten economic restrictions on Russia. [Read more about Britain’s options against Russia.]
Britain’s broadcast regulator has also hinted that it could revoke the license of RT, the Kremlin-controlled English-language news channel.
“I expected a stronger reaction,” said Jonathan Eyal, international director at the Royal United Services Institute, a London-based defense and security research institute. He said Mrs. May might have calculated that in response to tougher measures, Russia would “decapitate the British Embassy in Moscow” with its own expulsions.
Britain wants the support of its allies in taking action against Russia, but relations with those allies are shakier than they have been in generations, given Britain’s pending divorce from the European Union and frictions with Mr. Trump.
The European alliance itself is being sorely tested, with the rise of far-right and anti-Europe movements in Italy and elsewhere, defiance from autocratic sounding governments in Poland and Hungary, and the long struggle in Germany to form a governing coalition.
By contrast, the Kremlin sees the tide of events moving in its direction, and little likelihood that Western allies possess the combined will to increase sanctions.
“The Russian authorities don’t feel themselves isolated at all,” said Thomas Gomart, director of the French Institute of International Relations, a research group based in Paris.
“They feel there is disarray in the West because of the situation in Washington, because of Brexit, because of the Italian elections and the difficulty of forming a government in Berlin,” he added.
The Kremlin has denied any involvement in the nerve agent attack and vowed to retaliate for any measure Britain takes, and on Wednesday, the foreign minister, Sergey Lavrov, accused the British government of “acting out political drama” rather than conducting a serious investigation.
In Britain, Mrs. May leads a minority government, and her nation is mired in painful Brexit talks. Every week brings fresh speculation that her own bitterly divided party will dump her.
Outside of the rhetorical realm, it is not clear what degree of international support she can muster.
She may find it hard to win allies to put increased economic and security pressure on Russia, which is already under sanctions, though she lined up expressions of support this week from United Nations agencies, NATO, the European Union and elsewhere.
It is unclear why a few assassinations or attempts would spur allies to take tougher action against Moscow when killing civilians in Syria, shooting down a passenger airliner over Ukraine and meddling in the American election generally have not.
French officials have described the use of a nerve agent in Britain as an outrage, and Britain as a vital ally with shared values, but they have stopped short of blaming Russia or discussing any retaliatory action.
“Once elements are proven, then will come the time for decisions,” Benjamin Griveaux, the French government spokesman, told reporters after a cabinet meeting.
Angela Merkel, who was sworn in as German chancellor for the fourth time on Wednesday, said only that she took Britain’s conclusions seriously, and that more transparency was needed from Russia, according to the German news agency DPA.
Some of the strongest language came at an emergency session of the United Nations Security Council on Wednesday from the American ambassador, Nikki R. Haley, who has been tougher on Russia than Mr. Trump.
“Russia is responsible for the attack on two people in the United Kingdom using a military grade nerve agent,” Ms. Haley said, calling the poisoning “an atrocious crime.”
“We take no pleasure in having to constantly criticize Russia,” she said, “but we need Russia to stop giving us so many reasons to do so.”
Even so, analysts say, Britain’s friends would have little appetite for taking on a Russian government that has demonstrated a willingness to lash out with cyberwarfare,disinformation, assassinations and the manipulation of energy exports.
New sanctions are unlikely, Mr. Gomart said, but in Paris and Berlin, “the level of concern is higher than ever” over Russian behavior, and governments across the Continent will give Britain practical and intelligence support in facing down Moscow.
Mrs. May has said that Sergei V. Skripal, 66, and his daughter, Yulia S. Skripal, 33, were sickened on March 4 with a “novichok,” an extremely potent class of nerve agents developed by the Soviet Union in the 1970s and ’80s.
The Skripals, who remain hospitalized in critical condition, were poisoned in Salisbury, the small city where Mr. Skripal lives, potentially endangering hundreds of bystanders.
Mr. Skripal, a former colonel in Russian military intelligence, was imprisoned for selling secrets to Britain, and then sent to Britain in 2010 in a spy swap.
The prime minister said the British intelligence services have concluded that either Russia was behind the attack or that it had lost control of chemical weapons that, under an international treaty, it claims not to possess.
Mrs. May also said that the government had agreed on new powers to crack down on the activities of foreign intelligence agents in Britain, that there was no place for “serious criminals and corrupt elites” in the country, and that an invitation for Mr. Lavrov to visit had been withdrawn.
She added that no British ministers or royals would attend the World Cup in Russia this summer, that Britain would “increase checks on private flights, customs and freight,” and that it would “freeze Russian state assets wherever we have the evidence that they may be used to threaten the life or property of U.K. nationals or residents.”
“They have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt and defiance,” Mrs. May said of Russia. “Their response has demonstrated complete disdain for the gravity of these events. They have provided no credible explanation.”
Her tough stance won widespread praise in Britain, uniting her fractious Conservative Party — and much of the opposition, as well — for now.
A notable exception was Jeremy Corbyn, leader of the Labour Party, who left open the possibility that the nerve agent could have been used by someone other than the Russian state, and whose aide, Seamus Milne, referred to British intelligence failings over Iraqi weapons 15 years ago.
“This ghastly episode has put her in a position where her political interest, to show that she is an authoritative leader, coincides with the national interest,” said James Sherr, a former British defense official who is an associate fellow at Chatham House, a prominent think tank.
“But she also had no choice,” Mr. Sherr added. “The Russians have perceived this country for a long time as being very weak, and we have reinforced that perception with a very lackluster response to murders on our soil. She had to respond this time.”
Experts questioned how much substance lay behind the prime minister’s promises of action, and warned — as she has, herself — that going after Russia will hurt Britain, too.
Tens of billions of dollars in Russian wealth have poured into Britain in recent years, much of it invested by allies of Mr. Putin. D riving that money away could have painful consequences for London, on top of any range of economic setbacks that are expected to accompany Britain’s decision to leave the European Union.
Mrs. May promised to invoke a law allowing the government to investigate and seize assets gained from corruption, but that law has been in effect since last year and has been little used. She said the government would compile better records on corporate ownership used to hide assets, but that project, too, has been underway for years.
And she said that her government would propose new legislative changes to use against Mr. Putin’s allies, but offered few details.
“We will continue to bring all the capabilities of U.K. law enforcement to bear against serious criminals and corrupt elites,” she said. “There is no place for these people — or their money — in our country.”
But Mr. Sherr said that the real question “is whether there will be a serious, long-term strategy to keep up the pressure.”
“One or two headline-grabbing speeches and a few other measures are not going to change things,” he said.
Aurelien Breeden contributed reporting from Paris, Christopher Schuetze from Berlin, and Sophia Kishkovsky from Moscow.