The UK will expel 23 Russian diplomats after Moscow refused to explain how a Russian-made nerve agent was used to poison a former Russian double-agent spy in Salisbury, British Prime Minister Theresa May announced on Wednesday afternoon.

May said she and the British intelligence community had determined that Russia was responsible for the attempted murder.

Skripal, 66, and his 33-year-old daughter Yulia were found unconscious on a bench outside a shopping centre in Salisbury on March 4. They two remain in hospital and in critical condition.

May said the diplomats, who have a week to leave, were identified as “undeclared intelligence officers”.

“There is no alternative conclusion, other than that the Russian state was culpable for the attempted murder of Mr Skripal and his daughter, and for threatening the lives of other British citizens in Salisbury,” May said.

The UK has also called for an emergency UN Security Council meeting. Russia, which is a permanent member of the UN security council and has a right of veto, said the Kremlin would retaliate against any measures that the UK takes, saying the British government has only made “baseless accusations which are not backed by any evidence”.

The UK government could get tougher by seizing or freezing the assets of Putin-linked Russian oligarchs whose money has flooded the London property market since the oil-boom years of the mid-2000s.

Some experts have even suggested May could invoke NATO’s collective-defence clause, meaning allies such as the US and Germany would be asked to come to Britain’s aid.