The Prime Minister told the House of Commons she has summoned Russia’s ambassador to the Foreign Office to explain whether the attack was “a direct action by the Russian state” or whether it was a result of Moscow “losing control” of its stock of nerve agents.
Mrs May also revealed in the Commons that 66-year-old Mr Skripal and his 33-year-old daughter were poisoned with the nerve agent Novichok.
She said: “Based on the positive identification of this agent, our knowledge that Russia has previously produced this agent, its record of conducting state-sponsored assassinations abroad, and its view of of some defectors as legitimate targets for assassination…it is highly likely that Russia was responsible.
“Either this was a direct action by the Russian state against our country or the Russian government lost control of the agent and allowed it to get into the hands of others.”
The Prime Minister said that Foreign Secretary Boris Johnson has told the Russian ambassador that Moscow must “immediately provide full and complete disclosure” of its Novichok nerve gas programme.
“We saw promises by the Russian government to assist in the investigation but we were met instead by obfuscation.
“We must now stand ready to take much more considered action. This action amounts to an unlawful use of force by the Russian state against the United Kingdom.
“We will not accept such a brazen attempt to murder civilians on our soil.”
Former Conservative Party leader Iain Duncan Smith told Mrs May the Commons would “back her” to take “the most severe action as is required” against Moscow.
He added: “This country, Russia, is now as close to being a rogue state as any. It occupies Crimea, it has helped occupy eastern Ukraine, it has created hell on earth in Syria and is even now overseeing worse action.
“This is a country locking up its members of the opposition, it is a country, frankly, we’ve learned this lesson before, if we appease a country like this then we expect even worse.”
Mrs May responded: “He is absolutely right, nobody should be in any doubt of the various activities that the Russian state is involved in across the continent of Europe and elsewhere.
“It is important that this Government, that this country, stands up very clearly and not only calls out actions by Russia but also ensures that we have a robust response to them.”
It came as police in decontamination suits and military chemical weapons experts cordoned off part of a Wiltshire village about eight miles from Salisbury. Experts were pictured in the Winterslow village on Monday afternoon.
Local postmaster Bella Thomas said: “The press keep saying about sleepy Salisbury city centre, and we’re obviously a sleepy village, it’s a bit of a shock. It’s like a scene from the Twilight Zone, something you would see in the movies.”
She said that the massive police investigation, supported by the military, has crippled some businesses in the city centre.
“I do feel for the shopkeepers. They are really suffering. It’s not cheap having a business in town and it has crippled some of their businesses,” she said.
Police also sealed off the top level of the car park of a branch of Sainsbury’s supermarket, which is in the same shopping precinct where Mr Skripal and his daughter Yulia were found ill eight days ago.
At the weekend hundreds of diners and pub-goers who had been at the same venues as the Russian pair were urged to wash clothes and other items a week after potentially coming into contact with a Novichok nerve agent.
The “precautionary advice” was issued after traces of the substance were found in The Mill pub and the nearby Zizzi restaurant, in Salisbury.
The Russian government continued to deny any involvement in the attack in a statement released earlier on Monday.
A statement from the Russian Embassy reads: “The investigation is being concluded in a non-transparent manner, whenever for the Russian side nor for the public. Quite naturally, this is a source of concern.
“We would like to stress once again that we are outraged by the anti-Russian media campaign, condoned by the Government, that influences the investigation and has a psychological effect on British residents.
“Current policy of the UK Government towards Russia is a very dangerous game played with the British public opinion, which not only sends the investigation upon an unhelpful political track but also bears the risk of more serious long-term consequences for our relations.”