RUSSIA has refused to respond to Theresa May’s ultimatum regarding the nerve agent attack on an ex-Russian spy the country’s London embassy has confirmed, lurching the UK into a diplomatic crisis with Moscow.

The UK Prime Minister addressed the House of Commons on Monday and said it was “highly likely” Russia was responsible for the attack and if they fail to respond she will conclude it was an “unlawful use of force” by the Russian state against the UK.

Mrs May gave Russia until Midnight on Tuesday to explain how the USSR nerve agent ended up in the UK.

In a statement Russian Ambassador, Alexander Shulgin, said: “The British authorities’ unfounded accusations of Russia’s alleged involvement in using poisonous agents on their territory are absolutely unacceptable.

“Our British colleagues should recall that Russia and the United Kingdom are members of the Organisation for the Prohibition of Chemical Weapons which is one of the most successful and effective disarmament and non-proliferation mechanisms.

“We call upon them to abandon the language of ultimatums and threats and return to the legal framework of the chemical convention, which makes it possible to resolve this kind of situation.”

Former Russian double-agent Sergei Skripal and his daughter Yulia remain in critical conditions after coming into contact with a nerve agent called Novichok as they dined in Salisbury.

Amber Rudd has vowed the UK will send a “powerful response” to Russian President Vladimir Putin in retaliation for the attack.

It is unknown exactly how the UK will respond to Russia’s refusal to give an explanation for the attack.

Britain's options are being weighed up by the Government - with a cyber counter-strike said to be among the possible measures being considered, along with economic, financial and diplomatic action.

MPs have demanded in the House of Commons the TV license for Russia Today, Moscow’s media propaganda arm, be revoked and Boris Johnson has previously suggested England should boycott the World Cup.

The Prime Minister has already received broad support over the chemical attack with allies saying they will support the UK in their response to Russia.

The US Secretary of State, Rex Tillerson, said yesterday: “There is never a justification of this type of attack - the attempted murder of a private citizen on the soil of a sovereign nation - and we are outraged that Russia appears to have engaged in such behaviour.”

Meanwhile, Number 10 announced that France has also said they will co-operate a response to the attack.

A Downing Street spokesman said: “They discussed the wide pattern of aggressive Russian behaviour and agreed that it would be important to continue to act in concert with allies to address it.

“President Macron condemned the attack and offered his solidarity with the UK.

“They agreed that the French and British Governments should co-ordinate closely as the investigation developed and following Russia's response.”

The EU has also given their backing to Mrs May.

A spokeswoman for EU foreign policy chief Federica Mogherini said: “We stand with the UK in pursuit of justice in this case and are ready to offer support if necessary.”

Theresa May will chair a meeting of the national security council today to discuss the UK’s response.