Up to 1,700 British troops are to be deployed to Afghanistan, Defence Secretary Mr Geoff Hoon has revealed.
It comes after the US asked the UK to provide forces to join future operations against al-Qaeda and the Taliban.
Mr Hoon told MPs he had authorised deployment to Afghanistan of a full UK infantry battle group, built around 45 Commando, Royal Marines.
He said it would join a US-led brigade "forming a potent force ready to undertake such operations".
He said the force would go intially to Bagram, with the first members of 45 Commando Group on the ground within days and ready to commence offensive operations by mid April.
Mr Hoon told the Commons: "These troops are being deployed to Afghanistan to take part in war fighting operations. We will be asking them to risk their lives ... They may suffer casualties."
Mr Hoon told MPs it would be the UK's largest military deployment for combat operations since the Gulf conflict.
PA