Prime Minister Theresa May threw 23 Russian diplomats out of Britain in retaliation for the poisoning of a former spy and his daughter on U.K. soil, as she braced the country for further attacks.

May said the U.K. will move to freeze Russian state assets where necessary in response to what she called an “unlawful use of force” involving a weapons-grade nerve agent against the U.K. More steps will be taken against Vladimir Putin’s government in secret, she said, a hint that Britain could launch cyber-attacks on Kremlin interests.

Other wide-ranging responses, such as financial sanctions or preventing Russian banks from using the SWIFT international payments messaging system, require multilateral support and time.

The Russian Embassy in London denounced the expulsions — the biggest for more than 30 years — as “totally unacceptable, unjustified and shortsighted.” May warned the U.K. to expect further “provocations” from Putin.

The crisis is a key test for the British prime minister and comes at a critical time in the country’s history. As she navigates the U.K.’s exit from the European Union, May is trying to maintain close security ties with the bloc while seeking to preserve British influence around the world. With only a year left until Brexit, the extent which the premier’s international counterparts choose to support her will indicate how they weigh relations with the U.K.

The first use of a nerve agent on European soil since World War II is also a direct challenge to the wider Western alliance on how it responds to Putin, days before elections are almost certain to give him a fourth term as president.

May’s retaliation come after Russia refused to recognize a deadline of midnight Tuesday to provide an explanation for the nerve agent attack on Sergei Skripal and his daughter Yulia in southwest England. The pair remain in a critical condition.

“They have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt and defiance,” May told Parliament on Thursday. “There is no alternative conclusion other than that the Russian state was culpable.”

May outlined the action she will take against Putin’s administration, starting with the removal of “undeclared intelligence officers” based in London to “dismantle the Russian espionage network in the U.K.”

She gave 23 officials operating out of the Russian embassy one week’s notice to leave the U.K., saying these individuals were effectively spies.

“This will be the single biggest expulsion for over thirty years and it reflects the fact that this is not the first time that the Russian State has acted against our country,” May said. “Through these expulsions we will fundamentally degrade Russian intelligence capability in the U.K. for years to come — and if they seek to rebuild it, we will prevent them from doing so.”

Other steps May announced include:

– New laws to give security services the power to detain individuals suspected of “hostile state activity” at the British border, with further measures considered to stop foreign agents operating in the U.K.

– Increased checks on private flights, customs and freight

– Freezing 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”

– Police to target “serious criminals and corrupt elites.” May said: “There is no place for these people – or their money – in our country.” This was the closest May came to threatening the wealth of London-based oligarchs — some of whom are Putin’s enemies while others are close to him

– High-level boycott of this year’s soccer World Cup in Russia — U.K. ministers and members of the British royal family will not attend

May did not make any mention of restricting the activities of Russian banks, or revoking visas for individual Russian citizens linked to the Kremlin. She said a decision on whether action should be taken against Russian broadcaster RT would be left to the regulator — the Kremlin had threatened to expel British journalists from Russia if the U.K. closed down RT.

“All the responsibility for the deterioration of the Russia-U.K. relationship lies with the current political leadership of Britain,” the Russian Embassy said.

The nerve agent used in the attack, called Novichok, was developed by the Soviet Union in the 1970s and 80s. The Skripal case also bears similarities to the 2006 murder of Alexander Litvinenko, a dissident Russian ex-spy who died after drinking tea spiked with radioactive polonium.

The prime minister said she’s seeking support from international allies and has already spoken to U.S. President Donald Trump, German Chancellor Angela Merkel and Emmanuel Macron, the French president.

Merkel said Russia must reveal what it knows about the poisoning. “Beyond this case, which on its own must be taken seriously, we have a number of conflicts with Russia,” she said in an interview with television broadcaster ARD, though she warned against breaking off all contact with the Kremlin.

Britain will be making its case for coordinated action at the United Nations Security Council later on Wednesday. Russia is one of five veto-wielding members on it.

“It’s unpleasant, but not the end of the world,” said Andrei Kortunov, head of the Russian International Affairs Council, a research group set up by the Kremlin. “There were much tougher steps mooted, up to a cyber attack. That would have been very dangerous. These measures don’t rule out dialogue which means they’re keeping the bridges open.”

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Bloomberg’s Jessica Shankleman, Thomas Penny, Henry Meyer and Ilya Arkhipov contributed.