Russia will not respond to Theresa May's midnight ultimatum over the poisoning of a former spy in Salisbury, the Russian foreign ministry has said, unless it is given samples of the nerve agent used in the attack.

In a statement, the foreign ministry said British threats to punish Moscow over the poisoning would not go unanswered, and described the allegations of Russian involvement as a provocation.

The UK Prime Minister had given Russia until midnight on Tuesday to explain why a Soviet-era nerve agent, produced in Russia, was used in the attack on former spy Sergei Skripal and his daughter Yulia, who remain in a serious condition in hospital.

Ms May said the only two explanations for the finding were that the attack was ordered by the Kremlin, or that the Russian government had lost control of the military grade chemical weapon and let it fall into the hands of the perpetrators.

Russia insists it had nothing to do with the attack and said it will ignore the ultimatum to explain itself until London both hands over samples of the Novichok agent used and begins to comply with its obligations under the Chemical Weapon Convention, which oversees joint investigations of such incidents.

"Any threats to take 'sanctions' against Russia will not be left without a response," the foreign ministry said in a statement. "The British side should understand that."

​Sergei Lavrov, the Russian Foreign Minister, said earlier on Tuesday that his country "is not to blame" for the poisoning of Sergei Skripal.

And 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.”

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

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.”

Boris Johnson, the UK Foreign Secretary, said Britain was talking to its allies about the situation and that he was "encouraged by the willingness of our friends to show support and solidarity".

Asked about the Salisbury attack, the US President Donald Trump told reporters outside the White House: "Well it sounds to me, I'm speaking to Theresa May today. It sounds to me like it would be Russia based on all of the evidence they have.

"I don't know if they've come to a conclusion, but she's calling me today."

He added: "Theresa May is going to be speaking to me today. It sounds to me like they believe it was Russia and I would certainly take that finding as fact."

Mr Trump added: "As soon as we get the facts straight and we are going to be speaking with the British today, we're speaking with Theresa May today, and as soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be.

"But I have not spoken to her, I'll speak to her sometime today."

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.

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."