Russia's foreign ministry has summoned Britain's ambassador to the country, Laurie Bristow, for a meeting. Russian Foreign Minister Sergei Lavrov said on Friday that his country is preparing to expel British diplomats. This is in retaliation to London's decision to expel 23 staff at the Russian embassy over the poisoning of an ex-Russian spy and his daughter. Russia has denied any involvement in the attempted murder. Relations between the two countries have crashed to a post-Cold War low as the geopolitical dispute over the nerve agent attack has escalated.