London (CNN) The UK will expel 23 Russian diplomats from the country after concluding that the Russian state is responsible for the attempted murder of a former Russian spy and his daughter on British soil, a step that marks a serious escalation of hostilities between London and Moscow.

This will be the single biggest expulsion of Russian diplomats in more than 30 years, Prime Minister Theresa May said Wednesday, as she set out a raft of measures intended to target Russian assets and strengthen UK defenses.

The 23 diplomats, who May said had been identified as undeclared intelligence officers, will have only one week to leave.

"For those who seek to do us harm, my message is simple. You are not welcome here," she said in a statement to the House of Commons following a meeting of Britain's National Security Council.

The expulsion of diplomats will "fundamentally degrade Russian intelligence capability in the UK for years to come," May said. All planned high-level bilateral contacts will also be suspended, she said.

The move comes after the poisoning of former Russian spy Sergei Skripal and his daughter Yulia in the southern English city of Salisbury on March 4. The Skripals are critically ill in the hospital after being exposed to a nerve agent, known as Novichok and developed in Russia. Thirty-six other people in Salisbury were seen by medics after the exposure. One, a police officer, remains hospitalized.

May said Monday it was highly likely that Moscow was behind the poisoning. The Russian ambassador to the UK was summoned to the UK Foreign Office to explain whether the attack was directed by Russian authorities, or whether Moscow had lost control of the nerve agent.

May demanded that the Kremlin respond by midnight Tuesday, London time, to the UK government's conclusion that Russia was linked to the poisoning of the Skripals.

But Moscow -- which has repeatedly dismissed any accusations of involvement in the attack -- ignored the deadline.

Following May's address, the Russian Embassy in London condemned the expulsion of its diplomats as a "hostile action" that is "totally unacceptable, unjustified and short-sighted" in a statement on its official website.

"All the responsibility for the deterioration of the Russia-UK relationship lies with the current political leadership of Britain," it said.

The Russian Foreign Ministry condemned the UK move as "an unprecedented gross provocation that undermines the foundations of a normal interstate dialogue between our countries."

In a statement, it claimed the British government had "made a choice in favor of confrontation with Russia" rather than completing its own investigation, and said it considered the "hostile measures" announced by May to be "categorically unacceptable and unworthy."

The United Nations Security Council has called an emergency session at 3 p.m. ET to discuss the Salisbury attack.

May: 'Full and robust response'

Addressing lawmakers Wednesday, May said it had been right to give Russia the opportunity to answer the allegations.

"But their response has demonstrated complete disdain for the gravity of these events," she said. "They have provided no credible explanation that could suggest they lost control of their nerve agent. No explanation as to how this agent came to be used in the United Kingdom; no explanation as to why Russia has an undeclared chemical weapons program in contravention of international law.

"Instead they have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt and defiance."

This leads to "no alternative conclusion," May said, than that the Russian state was culpable for the attempted murder of the Skripals and for threatening the lives of other British citizens.

"It must therefore be met with a full and robust response -- beyond the actions we have already taken since the murder of Mr. Litvinenko and to counter this pattern of Russian aggression elsewhere," she added, referring to another ex-Russian spy -- Alexander Litvinenko -- who was fatally poisoned in London in 2006.

May said she would propose new legislative powers to strengthen Britain's defenses against hostile state activity and target those responsible for human-rights abuses through sanctions.

Britain will also step up its efforts to monitor those traveling to the UK who may pose a security threat, she said, and crack down on "corrupt elites." She also announced plans to freeze Russian state assets "wherever we have the evidence that they may be used to threaten the life or property of UK nationals or residents," she said.

Military personnel wearing protective suits remove a police car and other vehicles from a public parking lot in Salisbury on Sunday.

As part of the suspension of high-level contacts, no UK ministers or royals will attend the soccer World Cup in Russia this summer and the UK has revoked an invitation to Russian Foreign Minister Sergey Lavrov to visit Britain, May said.

The Prime Minister stressed that she had the support of international partners, including NATO and the European Union. She also insisted that she did not blame the Russian people for events in Salisbury, pointing instead toward Russian President Vladimir Putin.

"Many of us looked at a post-Soviet Russia with hope. We wanted a better relationship and it is tragic that President Putin has chosen to act in this way," she said.

"But we will not tolerate the threat to life of British people and others on British soil from the Russian Government. Nor will we tolerate such a flagrant breach of Russia's international obligations."

NATO on Wednesday expressed "deep concern " over the Salisbury incident, saying that it was "the first offensive use of a nerve agent on alliance territory since NATO's foundation."

It urged Russia to address Britain's questions, including "providing full and complete disclosure" of the Novichok program to the Organisation for the Prohibition of Chemical Weapons, the body that implements the Chemical Weapons Convention.

Kremlin: 'Unfounded accusations'

Speaking in Moscow earlier Wednesday, Lavrov accused UK authorities of "political theater" and trying to mislead the international community rather than submitting an official request to Russia in accordance with the Chemical Weapons Convention. Russia is ready to respond to an official request within 10 days, in line with its obligations, once it is made, Lavrov said.

Lavrov also said that Russia had no motive in targeting the specific individuals concerned and that all allegations were based on speculation.

Kremlin spokesman Dmitry Peskov similarly rejected "unfounded accusations" of Russian involvement in the attack as he spoke with reporters on a conference call. He urged other countries to use their "common sense" as they consider whether there is any proof to the claims.

Skripal is believed to have lived in the UK since his release from Russian custody in 2010.

He was convicted in Russia of spying for Britain before he was granted asylum in the UK after a high-profile spy swap in 2010 between the US and Russia.

Glushkov had links to compatriots who died in unexplained circumstances in the UK, but police said there was no evidence to suggest a link between Glushkov's death and Skripal's poisoning.