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MOSCOW (Reuters) - Russia’s foreign ministry summoned Britain’s ambassador to the country, Laurie Bristow, for a meeting on Saturday, Russian news agencies reported, citing an unidentified ministry source.

Russia is expected to expel British diplomats in retaliation for Prime Minister Theresa May’s decision to kick out 23 Russians, as relations between the two countries crash to a post-Cold War low over an attack involving a military-grade nerve agent on English soil.

Russian Foreign Ministry spokeswoman Maria Zakharova said on Friday Moscow had already decided on retaliatory measures, which she said Britain would be informed of in the near future.

After the first known offensive use of a nerve agent in Europe since World War Two, Britain has pointed the finger at President Vladimir Putin and on Thursday May gave the 23 Russians - who she said were spies working under diplomatic cover at the embassy in London - a week to leave.

Moscow has denied any involvement in the attempted murder of former Russian double agent Sergei Skripal and his daughter. It has cast Britain as a post-colonial power unsettled by its impending exit from the European Union, and even suggested London fabricated the attack to whip up anti-Russian hysteria.

Asked by a Reuters reporter in the Kazakh capital if Moscow would expel British diplomats, Russian Foreign Minister Sergei Lavrov smiled and said on Friday: “We will, of course.”