Giving them a week's time to leave Britain, May has declared these diplomats as undeclared intelligence officers.

May said that an invitation to Russia's foreign minister has also been revoked, while adding that the British Royal Family would not attend the FIFA World Cup in Russia later this year.

Later, Britain's deputy UN ambassador Jonathan Allen told the UN Security Council that Russia has violated the Convention on the Prohibition of Chemical Weapons.

"We will stand by the values which are shared by the overwhelming majority of those in this council in this United Nations and we ask you today, to stand by us," he said.

Meanwhile, Moscow has denied the allegations and said it will take appropriate steps.

Russian ambassador to the UN, Vasily Nebenzya, demanded proof from the British government based on which it has levied the charges.

"We were given an ultimatum and requested in 24 hours to admit that we committed a crime. In other words, confess," he said. "We do not speak the language of ultimatums. We do not use that language with anyone. And we will not allow to be spoken to in that language either."

According to the new order, checks on private flights, customs and freight will be increased.

Certain Russian assets in the UK will also be freezed.

All planned high-level bilateral contacts between London and Moscow have been suspended.