The UK has called for an urgent meeting of the UN Security Council to discuss the poisoning of a former double agent in Salisbury, the Foreign Office says.
Russia failed to respond to a midnight deadline set by Theresa May to explain how a Russian-made nerve agent was used on Sergei Skripal and daughter Yulia.
The PM is set to announce a series of measures against Russia ahead the UN meeting, expected this evening.
Russia said it had “no motive” in the attempted murder of the pair.
The Kremlin, which has consistently denied involvement, said any steps by the PM would lead to “retaliatory measures”.
Mrs May, who has been briefed by senior intelligence chiefs in Downing Street, is due to give an update to MPs.
Earlier, the UK briefed Nato’s North Atlantic Council on the incident, which Britain described as “indiscriminate and reckless”.
At the meeting, Nato allies expressed “deep concern” at the use of a nerve agent and said it was a “clear breach of international norms and agreements”.
The Foreign Office said the incident in Salisbury on 4 March, which left the Skripals critically ill, “follows a well-established pattern of Russian state aggression”.
Det Sgt Nick Bailey also fell ill responding to the incident, and is in a serious but stable condition, but is thought to be improving.