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.
The latest WikiLeaks release comes after relations between Moscow and London soured as a result of Britain's decision to expel a Russian parliamentary researcher suspected of being a spy.
The memo, written by staff at the US embassy in Paris, records "an amicable 7 December dinner meeting with ambassador-at-large Henry Crumpton [and] Russian special presidential representative Anatoliy Safonov", two weeks after Litvinenko's death from polonium poisoning had triggered an international hunt for his killers.
WikiLeaks cables on guardian.co.uk
Also see 'Russian state killed former spy Alexander Litvinenko' - The Telegraph