Europe

Britain, Russia brace for poison showdown

Theresa May has set a deadline for Russian answers over the attack in Salisbury. (AAP)

Russia is not responding to British PM Theresa May's ultimatum about how a Soviet-era nerve toxin was used to strike down a former Russian double agent.

Britain has braced for a showdown with Russia 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 voiced support for 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, in effect challenging Britain to show what sanctions it would impose against Russian interests.

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 she chaired a meeting of the National Security Council at her Downing Street office in the morning.

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 centre, 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, himself a former KGB spy, 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.

US President Donald 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."

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