UK Prime Minister May said it was "highly likely" Russia was responsible for the Salisbury attack.
The Foreign Office summoned Russia's ambassador to provide an explanation.
PM May said if there is no "credible response" by the end of Tuesday, the UK would conclude there has been an "unlawful use of force" by Moscow.
The chemical used in the attack, the PM said, has been identified as one of a group of nerve agents known as Novichok.
US Secretary of State Rex Tillerson on Monday slammed Russia over an alleged nerve agent attack in Britain — going further than the White House in directly calling out the Kremlin as “an irresponsible force of instability in the world.”
Tillerson said the United States will stand “in solidarity” with the United Kingdom and would coordinate its response to the attack. At least two people, former Russian spy Sergei Skripal and his daughter Yulia, have been left extremely sick due to the alleged nerve agent.
NATO Article 5 says :
Looks like the UK is brave enough to stand up to Russia.The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.