LONDON - Britain ordered the expulsion of 23 Russian diplomats believed involved in espionage-related activities, British Prime Minister announced Wednesday, in the first wave of measures against Moscow for a nerve-gas attack against a former double agent.May, speaking to Parliament, also outlined a range of other steps, including a halt to high-level meetings with Russian officials and calling off a planned visit to Britain by Russian Foreign Minister Sergey Lavrov.May repeated the conclusion of British investigators that Russia had either deployed or lost control of dangerous nerve agents used in the attack - targeting the former spy and his daughter - and called Russia’s defiant response has "demonstrated complete disdain for the gravity of these events.""Instead they have treated the use of a military grade nerve agent in Europe with sarcasm, contempt and defiance," she told lawmakers while announcing the reprisal measures.She gave no further details on the Russian diplomats ordered expelled, but said they were deemed "undeclared intelligence officers." She called it the largest expulsion of Russian diplomats from Britain since Cold War-era retribution in the 1970s.May said more countermeasures were being considered. She said Britain sought support from the United States, European Union and NATO, but did not outline any requests she made from allies to join in the reprisals.Lawmakers in Parliament asked May pointedly what Britain’s allies were willing to do - and she mostly evaded the question, except to say that they had offered Britain support.Read More: Putin appears to target Tampa during speech touting advancements in Russia’s nukesEarlier, Britain’s Foreign Ministry also called for an urgent meeting of the U.N. Security Council to update members on the investigation into the nerve agent attack. Russia, as part of the permanent five nations on the council, holds veto power over any possible U.N. moves to come."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," May said in Parliament.On Monday, May asserted that it was "highly likely" that Russia was behind a poison attack and gave the Russian government a deadline to explain itself and where the rare and powerful "weapons-grade" nerve agent came from.As expected, May’s deadline passed on Wednesday and Russia did not respond - or did not respond with the details or explanations that Britain sought.Instead, Russian officials and state media assailed the British for whipping up "anti-Russia hysteria." The Kremlin rejected the "unfounded accusations" and shrugged off British demands.British politicians and commentators said May could employ a range of diplomatic and financial sanctions - from clamping down on Russian oligarchs’ property-buying binge in London to tossing out embassy staff.May could also ask the European Union, or even NATO, to join in a response to what she described as a "reckless" and "indiscriminate" attack, which not only endangered the lives of its two principal victims, the former Russian spy Sergei Skripal, 66, and his daughter, Yulia, 33, but also potentially exposed scores of others to the nerve agent, including a police officer who remains hospitalized.Skripal was jailed in Russia in 2006 for selling state secrets to British intelligence for 10 years, but he was released in 2010 as part of a high-profile spy swap. He and his daughter remain in critical condition at a Salisbury hospital.A spokesperson for 10 Downing Street said the British leader discussed the attack with President Donald Trump, who said Washington was "with the U.K. all the way" that Russia "must provide unambiguous answers as to how this nerve agent came to be used."In his last remarks, just hours after being fired by Trump via Twitter, Secretary of State Rex Tillerson warned, "much work remains to respond to the troubling behavior and actions on the part of the Russian government."Tillerson warned, "Russia must assess carefully as to how its actions are in the best interest of the Russian people and of the world more broadly. Continuing on their current trajectory is likely to lead to greater isolation on their part, a situation which is not in anyone’s interest."May also spoke with German Chancellor Angela Merkel. "They agreed that the international community should coordinate closely as the investigation developed and in the wake of Russia’s response," said her spokesman.Russia essentially blew off May’s midnight deadline for an explanation of how deadly Novichok nerve gas appeared on the streets of the quiet medieval town of Salisbury, famous for its nearby ruins of Stonehenge.Various officials and commentators made it clear that Moscow would call her bluff.After Russian Foreign Minister Lavrov told Britain on Tuesday that Moscow had no intention of responding to May’s ultimatum, the ministry’s spokeswoman, Maria Zakharova, dialed up the heat on an evening talk show on one of the leading state-run channels."No one can come before their national parliament and say: I give Russia 24 hours," said Zakharova, "What kind of conversation is that in principle?" She then appeared to chastise London for not behaving like a nuclear power, and took a shot at Boris Johnson."When a country’s Foreign Ministry is led by people who have absolutely nothing to do with foreign policy, who have built their career around populism, they have no idea either about the organization for the prohibition of nuclear weapons or the relevant [chemical weapons convention].""To them, it is normal to go out and start intimidating," Zakharova said. "Don’t. There is no need."In a conference call with journalists in Moscow Wednesday, Kremlin spokesman Dmitry Peskov said that theories surrounding Skripal’s poisoning are not the Kremlin’s problem. He reiterated that Moscow’s official position, that it was not involved and demands proof, has been delivered through diplomatic channels.Peskov said that Moscow does not accept London’s accusations and hopes the West will come to their senses and engage Russia in a joint investigation into the poisoning of Skripal.Regarding possible British actions against Russia today, Peskov said that "any unlawful actions against any Russian media outlets in the UK will, of course, lead to reciprocal measures backed the principle of reciprocity." So far, no one in Russia has specified which outlets a response would apply to, thought they have suggested that every British outlet could be targeted.So far, Foreign Minister Lavrov is the most senior Russian official to comment on May’s ultimatum. President Vladimir Putin, according to his press service, was traveling to Russia’s southern Dagestan Republic.