LONDON, March 14 (Reuters) - Britain 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. The United States, European Union and NATO supported Britain after May said it was "highly likely" that Russia was behind the poisoning of Sergei Skripal and his daughter with Novichok, a nerve agent developed by the Soviet military. Russia, which denied any involvement, said it was not responding to May's ultimatum until it received samples of the nerve agent, effectively challenging Britain to show what sanctions it would impose against Russian interests. "Moscow will not respond to London's ultimatum until it receives samples of the chemical substance," Russia's embassy in London said. "Any threat to take punitive measures against Russia will meet with a response." A spokeswoman for the British Foreign Office declined immediate comment. Britain 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. It may also cut back participation in the soccer World Cup, which Russia is hosting in June and July. Russia is due to hold a presidential election on Sunday in which Vladimir Putin is expected to coast to a fourth term in the Kremlin. He was first installed as Kremlin chief by Boris Yeltsin on the last day of 1999. U.S. President Donald Trump told May by telephone that 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." May is expected to outline Britain's response in the British parliament on Wednesday after a meeting of Britain's National Security Council. 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.
