Russia has vowed to hit back after Theresa May announced she is expelling 23 of the country’s diplomats in retaliation over the nerve agent attack on a former spy.

The nation warned its response "will not be long in coming” after Prime Minister Mrs May declared that the UK will impose new sanctions on the Kremlin, which Russia described as “absolutely unacceptable”.

The threat from the Russian foreign ministry came amid reports that British officials would be expelled from Moscow as part of its retaliation following Wednesday’s developments in the Commons.

Mrs May today addressed MPs after Vladimir Putin failed to meet the deadline she imposed to explain how a Russian-made nerve agent was used on Sergei Sergei Skripal, 66, and his daughter Yulia, 33, in Salisbury.

She chaired a meeting of the National Security Council as minister Sajid Javid accused Russia of a “brazen attempt to murder civilians on British soil”.

It comes as the Russian ambassador to the UN said Russia had “nothing to do with this incident” adding that the nation has “nothing to fear. Nothing to hide”.

And in one sarcastic aside, the Russian representative compared the UK to the fictional Inspector Lestrade, saying the UN "needs a Sherlock Holmes" to resolve the matter.

Speaking at a meeting of the UN Security Council this evening, he said: "Russia had nothing to do with this incident. The ultimatum from London is something that we cannot pay attention to and we expect that the UK will act in strict adherence to the other instruments including the European Convention on Legal Assistance in Criminal Matters.

"This is the only civilized way of settling the issue."

Military personnel wearing protective suits remove a police car and other vehicles from a public car park as they continue investigations into the poisoning of Sergei Skripal on March 11, 2018 in Salisbury, England. Chris J Ratcliffe/Getty

Russian President Vladimir Putin’s spokesman had earlier said his country had “no connection” to the poisoning and “won’t accept the language of ultimatum” from the UK.

And in its latest statement, the Russian foreign ministry described Mrs May’s measures as an “unprecedented, flagrant provocation that undermines the foundations of normal dialogue between our countries”.

It added: “Instead of completing its own investigation and using established international formats and instruments, including within the framework of the Organisation for the Prohibition of Chemical Weapons – in which we were prepared to cooperate – the British Government opted for confrontation with Russia. Obviously, by investigating this incident in a unilateral, non-transparent way, the British Government is again seeking to launch a groundless anti-Russian campaign.”

“Needless to say, our response measures will not be long in coming.”

Amid the rising tensions, the UK’s Foreign Office issued a warning to British nationals travelling to Russia as the row over the nerve attack on spy Sergei Skripal continued.

Britons currently in Russia or due to travel there in the coming weeks should be aware of "the possibility of anti-British sentiment or harassment", the FCO said.

The advice urged Brits to avoid any protests or demonstrations while in the country, and to monitor the ongoing political tensions closely.

At a meeting of the UN Security Council called to discuss the incident in Salisbury, Jonathan Allen, the UK’s deputy permanent representative to the UN, said that the Britain did not “jump to conclusions” in its update earlier today.

He said Russia has behaved in the same way it always has when flouting international law, using denial and distraction.

The government is asking the international chemical weapons watchdog to independently verify its analysis that a military-grade nerve agent from the former Soviet Union was used to poison Sergei and Yulia Skripal, Mr Allen said.

And he added that without any alternative explanation from Russian authorities about the nerve agent "we have no choice but to conclude this was a state-sponsored act against the prohibition and use of chemical weapons and in defiance of international law."

He said the Organisation for the Prohibition of Chemical Weapons has been informed about the use of the nerve agent and the UN "are inviting them to independently verify our analysis." He said: "We are making every effort to expedite this process."

Mr Allen described the attack as a “reckless act carried out by people who disregard the sanctity of human life”.

Theresa May. REUTERS/Toby Melville

The US ambassador to the world body Nikki Haley said: "Let me make one thing clear from the very beginning, the United States stands in absolute solidarity with Great Britain.

"The United States believes that Russia is responsible for the attack on two people in the United Kingdom using a military-grade nerve agent. Dozens of civilians and first responders were also exposed.

"No two nations enjoy a stronger bond than that of the United States and the United Kingdom. Ours is truly a special relationship. When our friends in Great Britain face a challenge, the United States will always be there for them. Always."

It came after a group of Labour backbenchers has said it "unequivocally accepts" the Russian state's culpability for the Salisbury attack after Jeremy Corbyn's team raised doubts about who was responsible.

The Labour leader's spokesman had said there was a "problematic" history over the use of UK intelligence and left open the possibility of the Federation being framed.

The spokesman indicated that the Labour party does not believe there is enough evidence yet to blame the Russian state for the attack.

Theresa May condemned the comments as "shocking" and "outrageous" after a Conservative MP raised them in the Commons chamber.

And the spokesman's comments prompted Labour backbencher John Woodcock to table an Early Day Motion "unequivocally" accepting the "Russian state's culpability" for the attack, and supporting "fully" the statement made by Mrs May in the Commons.

The motion was swiftly signed by a number of prominent critics of Mr Corbyn.