RUSSIA has said it "will not respond" to an "ultimatum" issued by the British government unless its terms are met. 

In a series of Tweets sent just before 5.30pm, the Russian Embassy said: "Moscow will not respond to London's ultimatum until it receives samples of the chemical substance to which the UK investigators are referring."

The Embassy called for Britain to launch a joint investigation to "comply with the Chemical Weapons Convention", adding: "Moscow is ready."

It said without a joint investigation, "the incident appears to be yet another crooked attempt by the UK authorities to discredit Russia".

It also said: "Any threat to take 'punitive' measures against Russia will meet with a response. The British side should be aware of that."

The Embassy today sent a note to the Foreign Office outlining demands for a joint investigation.  

It comes after Theresa May told MPs the nerve agent used to attack Sergei and Yulia Skripal was Novichok, a military grade agent of Russian origin. 

The prime minister said: "She said: "Either this was a direct act by the Russian state against our country or the Russian government lost control of its potentially catastrophically damaging nerve agent and it got into the hands of others."

Mrs May had summoned Russia's ambassador in London to the Foreign Office to explain whether the attack was planned by the Russian state. 

She said if a there was not a "satisfactory response" from the ambassador, "we will conclude that this action amounts to an unlawful use of force by the Russian state against the UK, and I will come back and set out the response".

Mrs May had given the state until the end of today to respond.