Britain summons Russian ambassador: government source

LONDON (Reuters) - Britain has summoned Russia's ambassador to the Foreign office in London, a government source said on Wednesday ahead of a statement by Prime Minister Theresa May on the use of a Soviet-era nerve toxin to attack a Russian former spy.

May was due to announce what Britain would do against Russia after a midnight deadline she set expired without an explanation from Moscow about the poisoning in the southern English city of Salisbury on March 4.

(Reporting by William James, editing by Elizabeth Piper)