Russia was tracking the assassins of dissident spy Alexander Litvinenko before he was poisoned but was warned off by Britain, which said the situation was “under control”, according to claims made in a leaked US diplomatic cable.

The secret memo, recording a 2006 meeting between an ex-CIA bureau chief and a former KGB officer, is set to reignite the diplomatic row surrounding Litvinenko’s unsolved murder that year, which many espionage experts have linked directly to the Kremlin.

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