Update 3:

Some more soundbites out of Russia, via Reuters:

RUSSIAN FOREIGN MINISTRY SAYS BRITISH GOVERNMENT HAS OPTED FOR FURTHER ESCALATION BY EXPELLING RUSSIAN DIPLOMATS

RUSSIAN FOREIGN MINISTRY SAYS BRITISH PM STATEMENT IS A FLAGRANT PROVOCATION

RUSSIAN FOREIGN MINISTRY SAYS BRITISH GOVERNMENT HAS CHOSEN CONFRONTATION WITH RUSSIA

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Update 2:

The head of the Upper House of Parliament in Russia has called the expulsion of diplomats a "provocation". He has promised Russia will react in a "fast, tough and reciprocal way".

Separately, Angela Merkel said that the EU is united on Russia but must keep talking to Russia. The German chancellor said: "We take the findings of the British government very seriously ... We will present a common European view here.

"Nonetheless, I say we can't break off all contacts now. We must still talk with the Russians despite all differences of opinion."

Julian Assange has also chimed in:

Theresa May is in a difficult position. If the Kremlin was knowingly behind the claimed nerve agent attack on Sergei and Yulia Skripal then it is likely designed to provoke a UK response with Russian moves already gamed out. Only a highly creative response can avoid such a trap. — Julian Assange ⌛ (@JulianAssange) March 14, 2018

Whilst expelling suspected intelligence agents is showy deterrent (such agents apply pressure towards their government to not engage in action that causes them to booted out) it also has high costs: Better the agent you know. — Julian Assange ⌛ (@JulianAssange) March 14, 2018

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Update:

As previewed earlier, Theresa May announced that Britain will expel 23 Russian diplomats who are "undeclared intelligence officers", i.e., spies. The retaliation comes as part of a range of measures in response to the poisoning of former double agent Sergei Skripal.

Speaking to parliament, Theresa May said the Russian state was culpable in the nerve agent attack in Salisbury on former Russian double agent Sergei Skripal and his daughter Yulia and is the reason for the expulsion of 23 diplomats: "All who been identified as undeclared intelligence officers. They have just one week to leave," she said.

UK will expel 23 Russian diplomats over nerve agent poisoning #Salisbury pic.twitter.com/1geAdkrZ58 — Sky News (@SkyNews) March 14, 2018

"This will be the single biggest expulsion for over 30 years and it will reflect the fact that this is not the first time the Russian state has acted against our country" she added.

She says Russia's response "has shown complete disdain" and the country has offered no explanation for the Russian-made novichok nerve agent used in the attack. The PM says the matter has been treated with "sarcasm, contempt and defiance".

Russian response has demonstrated "complete disdain" for gravity of events in #Salisbury and they have provided "no credible explanation" says @Theresa_May pic.twitter.com/GYbS7W6Exi — Sky News (@SkyNews) March 14, 2018

Echoing what she said earlier, May said that "Either this was a direct act by the Russian state against our country, or conceivably the Russian government could have lost control of a military grade nerve agent" and added that "In the aftermath of this appalling act against our country, this relationship cannot be the same."

"Either this was a direct act by the Russian state against our country, or conceivably the Russian government could have lost control of a military grade nerve agent" - @Theresa_May on #Salisbury pic.twitter.com/57nZbjTK4q — Sky News (@SkyNews) March 14, 2018

The move was among a set of measures announced in retaliation for what Mrs May called the “highly likely” involvement of the Russian state in the poisoning of former spy Sergei Skripal and his daughter on British soil.

Other highlights from the retaliation include the suspension of some Russian assets, a quasi boycott for the world cup, where no UK officials will be present, a suspension of all high-level contact with Russia, as well as sanctions for human-rights violations.

MAY: U.K. TO EXPEL 23 RUSSIAN DIPLOMATS IN RESPONSE TO ATTACK

MAY: EXPELLED RUSSIAN DIPLOMATS ARE INTELLIGENCE OFFICERS

MAY: EXAMINING NEED FOR NEW COUNTER-ESPIONAGE POWERS

MAY: WILL LOOK INTO SANCTIONS FOR HUMAN-RIGHTS VIOLATIONS

MAY: WILL FREEZE RUSSIAN ASSETS WHERE POSSIBLE

MAY: WILL USE EXISTING POWERS TO MONITOR TRAVELERS TO U.K.

MAY: NOT IN OUR INTEREST TO BREAK ALL DIALOGUE WITH RUSSIA

MAY: WILL SUSPEND ALL HIGH-LEVEL CONTACT WITH RUSSIA

MAY: NO ATTENDANCE BY OFFICIALS AT WORLD CUP

MAY: NO MINISTERS, ROYAL FAMILY TO WORLD CUP

MAY: SAYS SOME MEASURES AGAINST RUSSIA IT CANNOT DISCLOSE

MAY: WON'T TOLERATE FLAGRANT BREACH OF RUSSIA'S OBLIGATIONS

MAY: WILL GET OPCW TO VERIFY U.K. ANALYSIS OF NERVE AGENT

Furthermore, May said the UK will examine the need for new "counter-espionage" powers and will deploy some measures against Russia which it cannot disclose, although as Bloomberg's Leonid Bershidsky notes, "a secret response, if one is implied, won't do May much good. What the public cannot see isn't happening."

As a reminder, following Litvinenko's death several years ago, the UK similarly expelled Russian diplomats, suspended security cooperation, broke off bilateral plans on visas, froze the assets of the suspects and put them on international extradition lists; which makes today's response comparable.

Commenting on the response, Julian Rimmer, a London-based emerging-markets trader at Investec, said earlier that "there is no way the investment case for Russia cannot be undermined by whatever constitutes a 'full range of measures' from the U.K. PM," and added that "One can dispute the relative impact of the measures once they have been announced, but the net effect, to a greater or lesser extent, can only be detrimental."

Sure enough, the ruble is sliding on the news.

As are Russian stocks.

Commenting on the action, the Russian ambassador to the UK warned that Britain should expect retaliation for diplomat expulsions Alexander Vladimirovich Yakovenko tells Sky's Senior Political Correspondent Jason Farrell the UK's actions are "unacceptable" and that Moscow considers the expulsion of Russian diplomats "a provocation".

He says the measures have "nothing to do with the situation that we have in Salisbury".

"This is a really serious provocation."

Sure enough, Interfax reports that a Russian senator is calling for even more British diplomats to be expelled than the 23 Russians the U.K. is ousting.

And just like that the Cold War has made another return, this time in Russian-UK relations.

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Courtesy of SkyNews here are the key excerpts from Theresa May's speech:

It was right to offer Russia the opportunity to provide an explanation But their response has demonstrated complete disdain for the gravity of these events. 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 programme in contravention of international law. Instead they have treated the use of a military grade nerve agent in Europe with sarcasm, contempt and defiance. There is no alternative conclusion other than that the Russian State was culpable for the attempted murder of Mr Skripal and his daughter - and for threatening the lives of other British citizens in Salisbury, including Detective Sergeant Nick Bailey. This represents an unlawful use of force by the Russian State against the United Kingdom. It has taken place against the backdrop of a well-established pattern of Russian State aggression across Europe and beyond. It must therefore be met with a full and robust response. It is essential that we now come together – with our allies - to defend our security, to stand up for our values and to send a clear message to those who would seek to undermine them. This morning I chaired a further meeting of the National Security Council, where we agreed immediate actions to dismantle the Russian espionage network in the UK… …urgent work to develop new powers to tackle all forms of hostile state activity and to ensure that those seeking to carry out such activity cannot enter the UK… …and additional steps to suspend all planned high-level contacts between the United Kingdom and the Russian Federation. Let me start with the immediate actions. Under the Vienna Convention, the United Kingdom will now expel 23 Russian diplomats who have been identified as undeclared intelligence officers. They have just one week to leave. 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. We will fundamentally degrade Russian intelligence capability We will urgently develop proposals for new legislative powers to harden our defences against all forms of hostile state activity. This will include the addition of a targeted power to detain those suspected of hostile state activity at the UK border. This power is currently only permitted in relation to those suspected of terrorism. And I have asked the Home Secretary to consider whether there is a need for new counter-espionage power We will also table a Government amendment to the Sanctions Bill to strengthen our powers to impose sanctions in response to the violation of human rights We will also make full use of existing powers to enhance our efforts to monitor and track the intentions of those travelling to the UK who could be engaged in activity that threatens the security of the UK and of our allies. So we will increase checks on private flights, customs and freight. We will 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. And led by the National Crime Agency, we will continue to bring all the capabilities of UK law enforcement to bear against serious criminals and corrupt elites. There is no place for these people – or their money - in our country. We have had a very simple approach to Russia: Engage but beware. And I continue to believe it is not in our national interest to break off all dialogue between the United Kingdom and the Russian Federation. But in the aftermath of this appalling act against our country, this relationship cannot be the same. So we will suspend all planned high level bi-lateral contacts between the United Kingdom and the Russian Federation. This includes revoking the invitation to Foreign Minister Lavrov to pay a reciprocal visit to the United Kingdom... …and confirming there will be no attendance by Ministers - or indeed Members of the Royal Family - at this Summer’s World Cup in Russia. There are some that cannot be shared publicly for reasons of National Security. And, of course, there are other measures we stand ready to deploy at any time, should we face further Russian provocation. 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. 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. This was not just an act of attempted murder in Salisbury – nor just an act against UK. It is an affront to the prohibition on the use of chemical weapons. And it is an affront to the rules based system on which we and our international partners depend. We will work with our allies and partners to confront such actions wherever they threaten our security, at home and abroad.

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Earlier:

The UK was braced for a showdown with Russia on Wednesday after a midnight deadline set by Prime Minister Theresa May expired without an explanation from Moscow about how a Soviet-era nerve toxin was used to strike down a former Russian double agent.

Russia, which denied any involvement in the poisoning of Sergei Skripal and his daughter with Novichok, a nerve agent developed by the Soviet military, said it was not responding to May’s ultimatum until it received samples of the nerve agent, in effect challenging Britain to show what sanctions it would impose against Russian interests.

“Moscow had nothing to do with what happened in Britain. It will not accept any totally unfounded accusations directed against it and will also not accept the language of ultimatums,” Kremlin spokesman Dmitry Peskov told reporters on Wednesday according to Reuters. He added, however, that Russia remained open to cooperating with Britain in investigating the poisoning, blaming the British authorities for refusing to share information.

Russia’s Interfax news agency reported the Russian embassy in London planned to ask for consular access to Yulia Skripal, Sergei’s daughter.

Britain’s response to the expiry of the deadline and lack of explanation from Moscow was expected to be announced by May in parliament later, after May convened a meeting of the National Security Council at her Downing Street office in the morning. Furthermore, Bloomberg reported that the U.K. has called for an urgent meeting of the UN Security Council to update Council members on the investigation into the nerve agent attack in Salisbury, the U.K. Foreign Office said in a tweet.

The UK has called for an urgent meeting of the UN Security Council to update Council members on the investigation into the nerve agent attack in Salisbury. pic.twitter.com/jFQ2HA4JV0 — Foreign Office 🇬🇧 (@foreignoffice) March 14, 2018

In retaliation, it is possible that London could call on Western allies for a coordinated response, freeze the assets of Russian business leaders and officials, limit their access to London’s financial center, expel diplomats and even launch targeted cyber attacks. Furthermore, as Boris Johnson threatened, the UK may also cut back participation in the soccer World Cup, which Russia is hosting in June and July.

Meanwhile, as Reuters notes, the UK has already started its retaliation:

BRITAIN TO EXPEL SIGNIFICANT NUMBER OF RUSSIAN DIPLOMATS THOUGH NOT AS MANY AS IN 1971 - SKY NEWS REPORTER SAYS

This is likely just the start.

The official residence of Russia's ambassador to Britain, in central London

On Tuesday, President Trump told May by telephone Russia “must provide unambiguous answers regarding how this chemical weapon, developed in Russia, came to be used in the United Kingdom,” the White House said. The White House said Trump and May “agreed on the need for consequences for those who use these heinous weapons in flagrant violation of international norms.”

A British readout of the conversation said, “President Trump said the US was with the UK all the way.”

As a reminder, Skripal, 66, and his daughter Yulia, 33, were found slumped unconscious on a bench outside a shopping center in the genteel southern English city of Salisbury on March 4. They have been in a critical condition in hospital ever since. British scientists identified the poison as a military-grade nerve agent from a group of chemicals known as Novichok, first developed in the Soviet Union in the 1970s and 1980s.

On Monday, Theresa May said either the Russian state had poisoned Skripal, a former Russian military intelligence officer, or Russia had somehow lost control of its chemical weapons. Putin said last year that it had destroyed its last stockpiles of such weapons.

May said Russia had shown a pattern of aggression including the annexation of Crimea and the murder of former KGB agent Alexander Litvinenko, who died in 2006 after drinking green tea laced with radioactive polonium-210.

A public inquiry found the killing of Litvinenko had probably been approved by Putin and carried out by two Russians, one of them a former KGB bodyguard who became a member of the Russian parliament. Both denied responsibility, as did Moscow.

Counter-terrorism officers began investigating the death of another Russian in Britain on Tuesday, although police said it was not thought to be linked to the attack on the Skripals. Nikolai Glushkov, 68, who was an associate of late tycoon Boris Berezovsky, was found dead on Monday. Berezovsky was found dead in March 2013 with a scarf tied around his neck in the bathroom of his luxury mansion west of London.

And now that the UK has formally commenced retaliation, all eyes are on the Kremlin and how Putin will respond.