The move is in reaction to Britian's earlier decision to expel 23 diplomats over the alleged use of a nerve agent against a former Russian military intelligence officer who worked for British intelligence.

Seen here in a file picture, Russia's Foreign Minister Sergey Lavrov has said the country will retaliate against Britain's decision to expel 23 diplomats.
Seen here in a file picture, Russia's Foreign Minister Sergey Lavrov has said the country will retaliate against Britain's decision to expel 23 diplomats. (AP)

Russia is set to expel British diplomats in retaliation for Prime Minister Theresa May's decision to kick 23 Russians out as relations with London crash to a post-Cold War low due to an attack with military-grade nerve agent on English soil.

After the first known offensive use of such a weapon in Europe since World War Two, May blamed Moscow and gave 23 Russians who she said were spies working under diplomatic cover at the London embassy a week to leave.

Russia has denied any involvement, cast Britain as a post-colonial power unsettled by Brexit, and even suggested London had fabricated the attack in an attempt to whip up anti-Russian hysteria.

Asked by a Reuters reporter in the Kazakh capital if Russia planned to expel British diplomats from Moscow, Russian Foreign Minister Sergey Lavrov smiled and said: "We will, of course."

But there was no immediate explanation of when this would happen or how many British diplomats would be expelled.

No explanation yet

Britain, the United States, Germany and France jointly called on Russia on Thursday to explain the attack. US President Donald Trump said it looked like the Russians were behind it.

Russia has refused Britain's demands to explain how Novichok, a nerve agent developed by the Soviet military, was used to strike down Sergei Skripal, 66, and his daughter Yulia, 33, in the southern English city of Salisbury.

Skripal, a former colonel in the GRU who betrayed dozens of Russian agents to British intelligence, and his daughter have been critically ill since March 4, when they were found unconscious on a bench.

A British policeman who was also poisoned when he went to help them is in a serious but stable condition.

President Vladimir Putin, a former KGB spy who is poised to win a fourth term in an election on Sunday, has so far only said publicly that Britain should get to the bottom of what has happened.

Source: Reuters