Whistleblowers' website WikiLeaks obtained 91,000 US military records giving a blow-by-blow account of fighting between January 2004 and December 2009.

The files revealed new details about civilian casualties, a secret special forces unit targeting insurgent leaders, and concerns that Pakistani intelligence could be supporting the Taliban.

WikiLeaks founder Julian Assange said there was "no reason" to doubt the reliability of the material.

Security Minister Baroness Neville-Jones, former chair of the UK's Joint Intelligence Committee, described the leak as "really serious stuff".

She told BBC Breakfast: "We don't know how they got that material - it may be a combination of leaking of documents, but also one strongly suspects they have hacked into systems as well.

"This is a very, very big story. But if you stop to think about it for a moment, military systems have to be secure because people's lives are at stake."

Former British commanders in Afghanistan warned that the leaks could compromise operational security but questioned how much had been learned from them.

Colonel Stuart Tootal, who commanded 3rd Battalion Parachute Regiment in Helmand Province in 2006, said the information "could impact on the security of our soldiers".

A Downing Street spokeswoman said: "We would lament all unauthorised releases of classified material."