LONDON - British Prime Minister Theresa May announced measures on Wednesday against Moscow over its failure to respond to a demand by London to explain how a Russian-made nerve agent was used in a recent attack in Salisbury.

Former spy Sergei Skripal, 66, and his daughter Yulia, 33, are in a critical condition in hospital after being exposed to a nerve agent earlier this month.

The Russian government has denied any involvement in the attack, and has threatened reprisals against Britain if measures are taken by the government.

May announced that 23 Russian diplomats will be expelled from Britain and told to leave within a week. She said it is the biggest expulsion from Britain in 30 years.

The prime minister said Britain will not break off diplomatic relations with Russia, but all high level bilateral contacts will be canceled.

She said no government ministers or members of the royal family will attend the World Cup being hosted by Russia this year.

Russia's Ambassador to Britain, Alexander Yakovenko, warned London earlier on Wednesday of reciprocal measures if Britain expelled Russian diplomats, the RIA news agency said.

Moscow said it would not comply with Britain's demands unless the government provides samples of the poison collected by investigators.

Russian President Vladimir Putin's spokesman Dmitry Peskov told reporters earlier on Wednesday that Russia "rejects the language of ultimatums".

Peskov said Britain has so far only offered "baseless accusations which are not backed up by any evidence".

Russia has claimed that the nerve agent could have come from another former Soviet country, pointing to Moscow's foe, Ukraine.

Vladimir Gutenev, a member of Russia's state commission for chemical disarmament, said Russia had scrapped its stockpile of Novichok, the nerve agent used against the Skripals.

On Wednesday, London also called for an urgent meeting of the UN Security Council to discuss the investigation.

Xinhua - Reuters - Ap - Afp