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Prime Minister David Cameron has authorised an unprecedented swapping of secrets with the Russians as they try to track down Islamic extremist bombers.

It follows attacks last week on a railway station and a crowded commuter bus in Volgograd. Thirtyfour people died and at least 28 were injured.

British agents working in Russia are now sharing confidential assessments on extremists.

"This is the first time intelligence sharing like this has ever been done," said an insider.

"The Brit agents have been told to disclose once top-secret files on Islamic terrorists' movements and communications to senior men from Putin's security services."

Insiders say that No.10 and the White House realise "all hell would break loose" if they do not pass on information that could stall another attack.

MI6 and America's CIA have decades of experience in highlydangerous penetration operations in southern Russia against Muslim groups and monitor their communications.

Britain's GCHQ has also played a key role in eavesdropping on assassins in the area.

President Putin US president Barack Obama has also agreed to share his country's secrets to help keep the Games safe.

Thousands of Brits, including VIP guests and royalty, are expected at Sochi on Russia's Black Sea coast for next month's Games.