British Prime Minister Theresa May says it's "highly likely" Russia was responsible for the poisoning of ex-spy Sergei Skripal.

May says the Russian ex-spy poisoned in England was exposed to a military-grade nerve agent of a type produced by Russia.

May says Russia's ambassador to the UK has been summoned to explain how a Russian nerve agent turned up in Salisbury, the English city where Skripal, 66, and his daughter Yulia, 33, were poisoned. They remain in a critical condition in hospital.

The pair were discovered in a comatose state on a bench near a shopping centre in the quiet English city last week. British authorities said the two had been targeted with a rare nerve agent. A total of 21 people have received medical treatment after the mysterious attack.

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The Prime Minister says if Moscow is proven to be behind the poisoning, her government will consider it an "unlawful use of force" by Russia. She labelled the attack a "reckless and despicable act".

May told MPs: "It is now clear that Mr Skripal and his daughter were poisoned with a military grade nerve agent of a type developed by Russia. This is part of a group of nerve agents known as novichok.

"Based on the positive identification of this chemical agent by world leading experts at the laboratory at Port Down, our knowledge that Russia has previously produced this agent and would still be capable of doing so, Russia's record of conducting state sponsored assassinations, and our assessment that Russia views defectors as a legitimate target for assassination the Government has concluded that it is highly likely Russia was responsible for the act against Sergei and Yulia Skripal."

May said the Government would consider its "detailed" retaliation to the attack tomorrow, after the Kremlin had been given time to respond to the evidence.

"Should there be no credible response we will conclude that his action amounts to an unlawful use of force by the Russian state against the United Kingdom, and I will come back to this House and set out the full range of measures that we will take in response," she said.

May said the Government would not accept such an attempt to "murder innocent civilians on our soil".

Earlier, Russian President Vladimir Putin said Britain should figure out what happened to ex-spy Sergei Skripal before blaming the poisoning on Russia.

Asked by a British reporter in southern Russia if Russia was behind the poisoning, Putin said in comments carried by Russian news wires on Monday: "You first get to the bottom of things over there, and after that we can discuss it."