Russia has told Theresa May it “is not to blame” for the nerve agent attack on former spy Sergei Skripal.

Sergei Lavrov, the country’s foreign minister, said Moscow had demanded access to samples of the nerve agent used to poison Mr Skripal and his daughter in Salisbury.

Mr Lavrov said the British government had refused to provide Moscow access to materials and samples related to the case, which he called a violation of the Chemical Weapons Convention.

Moscow was willing to cooperate with the probe but suggested the UK would be “better off” complying with its international obligations “before putting forward ultimatums,” Mr Lavrov added.

Russian news agencies reported that the foreign ministry had summoned the British ambassador in Moscow over the poisoning accusations.

On Monday, Ms May said Mr Skripal and his daughter, Yulia, had been poisoned with a military-grade nerve agent known as Novichok, which had been developed in the Soviet Union.

The Prime Minister said Russia has until the end of Tuesday to explain how the substance ended up in Britain, otherwise the attack would be interpreted as an act of military aggression.

Officials said Ms May was reviewing a range of economic and diplomatic measures in retaliation for the assault.

The British government will have understood that the Kremlin was unlikely to respond to Ms May’s ultimatum positively. Many in Moscow are already bracing themselves for that they see as an inevitable tightening of sanctions.

Skripal attack aftermath – in pictures Show all 15 1 /15 Skripal attack aftermath – in pictures Skripal attack aftermath – in pictures Scene of attack Members of the emergency services in hazard suits fix the tent over the bench where Sergei and Yulia Skripal were found unconscious on a park bench in Salisbury in March 2018. Getty Skripal attack aftermath – in pictures Victim - Sergei Skripal The retired Russian colonel and former double agent for MI6 was in a critical condition in hospital for more than two months after being exposed to novichok in Salisbury. He was given refuge in the UK after being jailed in Moscow for treason. Mr Skripal came to Britain as part of a high-profile “spy swap” in 2010 in which four men were exchanged for ten Russian "sleeper agents" in the US. In this image he is speaking to his lawyer from behind bars in Moscow in 2006. AP Skripal attack aftermath – in pictures Victim - Yulia Skripal Yulia Skripal was struck down by a novichok poison alongside her father Sergei. Facebook Skripal attack aftermath – in pictures Scene of attack A police officer stands guard outside a branch of the Italian chain restaurant Zizzi where the pair dined at before falling ill. It was boarded off whilst investigators worked on the building and later found traces of the chemical weapon within it. AFP/Getty Skripal attack aftermath – in pictures Scene of attack Large areas of central Salisbury were cordoned off by police following the discovery of the Skripals. Traces of nerve agent were also found in The Mill pub. PA Skripal attack aftermath – in pictures Victim - Nick Bailey Detective Sergeant Nick Bailey, rushed to the aid of the Russian ex-spy and his daughter who were targeted with a nerve agent. He was hospitalized after aiding them and didn't leave until three weeks after the attack. Wiltshire Police/Rex Skripal attack aftermath – in pictures Police investigation - Skripal’s home Police believe they were poisoned at home, and detectives found the highest concentration of novichok on the front door of Mr Skripal’s house. Getty Skripal attack aftermath – in pictures Theresa May visits scene of attack Britain's Prime Minister Theresa May spokes with Wiltshire Police's Chief Constable Kier Pritchard near where the Skripal's were found. Britain expelled 23 Russian diplomats over the nerve agent poisoning and suspended high-level contacts, including for the World Cup on March 14. Theresa May told parliament that Russia had failed to respond to her demand for an explanation on how a Soviet-designed chemical, Novichok, was used in Salisbury. AFP/Getty Skripal attack aftermath – in pictures Skripal days before attack Sergei Skripal days before he was exposed to Novichok, that has left him fighting for life. ITV News Skripal attack aftermath – in pictures Police investigation - military involvement British soldiers were deployed soon after the attack to help a counter-terrorism investigation into the nerve agent attack. One of the places they were asked to help out with was Skripal's home and it's surrounding. They were asked to remove a vehicle connected to the agent attack in Salisbury, from a residential street in Gillingham. AFP/Getty Skripal attack aftermath – in pictures Police investigation Personnel in protective coveralls and breathing equipment cover an ambulance with a tarpaulin at the Salisbury District Hospital. AFP/Getty Skripal attack aftermath – in pictures Police investigation The investigation extended to the grave of Sergei Skripal's son Alexander in London Road cemetery. Getty Skripal attack aftermath – in pictures Police investigation The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. EPA Skripal attack aftermath – in pictures Home Secretary visits scene of attack Home Secretary Amber Rudd visited the scene of the nerve agent attack at the Maltings shopping centre on 9 March. Getty Skripal attack aftermath – in pictures Yulia Skripal speaks for the first time Yulia Skripal, speaking for the first time, said she felt lucky to have survived the nerve agent attack in Salisbury which left her fighting for life. Ms Skripal said her life had been “turned upside down” by the assassination attempt. But the Russian national added she hoped to return to her homeland one day, despite the Kremlin being blamed for the attack. Reuters

Ms May told her regular weekly Cabinet meeting at 10 Downing Street there was ”no doubt of the severity of what had taken place in Salisbury, which was a reckless, indiscriminate and despicable act.”

She confirmed she will chair a meeting of the National Security Council on Wednesday to discuss the Russian response and will then inform the House of commons of any measures to be taken.

Responding to Mr Lavrov’s complaint the UK had not provided samples of the nerve agent, Ms May’s official spokesman said: ”The UK complies fully with all its obligations under the chemical weapons convention.

“Under the chemical weapons convention states have the mechanism to consult, but there is no requirement to do so.”

The police and MI5 will look into allegations of Russian state involvement in a string of deaths on UK soil, the Government announced.

The announcement by the Home Secretary, Amber Rudd, followed reports claiming US intelligence sources suspect as many as 14 people may have been assassinated in Britain by Russia’s security services or mafia groups.

Investigators in protective clothing remove a van from an address in Winterslow near Salisbury (Getty)

Sergei Stepashin, Vladimir Putin’s predecessor as FSB director and Prime Minister, also called for British authorities to hand over evidence.

“We have the relevant agreements to investigate this together,” he told the Interfax news agency.

Mr Stepashin said British security services may have been complicit in the poisoning — and were using it to undermine Russia ahead of Sunday’s presidential elections: “It seems obvious to me that this might be the primitive work of English security services. Tell me who needs this traitor in Russia?”

There could be another reason apart from elections, he added: “The World Cup is about to take start and the English hate us for the fact the competition is taking place in our country.”

Earlier in the day, Konstantin Kosachyov, head of the Committee for Foreign Affairs in the Russian upper house described British allegations as “maniacal.” Britain was well versed in blaming all kinds of “mortal sins” on Russia, he wrote on Facebook.

“Russia is being asked to justify itself even without evidence,” he said. “In queen of courts of Britain, this degradation is complete: the total presumption of guilt, when the neither court and nor prosecutor are asked to prove the case, but the accused.”

Military officials in protective clothing remove vehicles from a car park in Salisbury (EPA)

On Tuesday, Boris Johnson said Britain was talking to its international partners about the situation.

“I’ve been encouraged by the willingness of our friends to show support and solidarity,” the Foreign Secretary said.

“I think in particular from President [Emmanuel] Macron of France, I talked to Sigmar Gabriel my German counterpart, and from Washington where Rex Tillerson last night made it absolutely clear that he sees this as part of a pattern of disruptive behaviour... malign behaviour by Russia... the support for the reckless use of chemical weapons which stretches from Syria now to the streets of Salisbury.”

Mr Tillerson, the US Secretary of State, said Russia’s actions would “certainly trigger a response.”

He said it was “almost beyond comprehension” a government would use such a dangerous substance in a public place.

Police officers stand outside a Zizzi restaurant in Salisbury as it remains closed as investigations continue into the poisoning of Sergei Skripal (Chris J Ratcliffe/Getty Images) (Getty)

The chief of the world’s chemical weapons watchdog said those responsible “must be held accountable.”

Ahmet Uzumcu said Boris Johnson had called him on Monday evening to inform him of the results.

In a speech to the Executive Council of the Organisation for the Prohibition of Chemical Weapons, Mr Uzumcu said: ”It is extremely worrying that chemical agents are still being used to harm people. Those found responsible for this use must be held accountable for their actions.

Britain’s representative to the watchdog told the watchdog’s council it is “highly likely” Russia was involved in the attack “by failure to control its own materials or by design.”

Mr Wilson “the first offensive use of a nerve agent of any sort on European territory since World War II.

He added: ”This attempted murder, using a weapons-grade nerve agent in a British city, was not just a crime against the Skripals. It was an indiscriminate and reckless act against the UK, which put the lives of innocent civilians at risk.”