Mr Hoon told MPs about the future role of UK troops
In response to a US request the battle group, which is built around 45 Commando Royal Navy Marines, is heading to Afghanistan to battle the remnants of the Taleban and al-Qaeda forces.
Mr Hoon said: "This a powerful force - total up to 1,700 strong."
The British contingent will join a US-led brigade.
"These troops are being deployed to Afghanistan to take part in war fighting operations," he said.
"We will be asking them to risk their lives...they may suffer casualties."
'Continuing threat'
Parts of 45 Commando are already in the region aboard HMS Ocean, Mr Hoon said, adding that the remaining companies were in a state of readiness in Arbroath.
The defence secretary said that al-Qaeda continued to pose a threat to countries including the UK.
Mr Hoon's Conservative opposite number, Bernard Jenkin, said that his party would back the deployment - although he indicated that the announcement had come as something of a surprise.
A 1,500-strong security force of British troops are currently serving with the 4,500-strong International Security and Assistance Force (ISAF).
The new deployment of troops will be involved in combat.
The Defence Secretary told MPs that a German unit would formally assume command from the UK of the subordinate headquarters of ISAF in Kabul.
Positive talks
The Ministry of Defence (MoD) has said that negotiations are still ongoing for Turkey to take over Britain's primary role in ISAF.
Mr Hoon told MPs that talks between Turkey, the UK and the US had been "extremely positive".
In January, Prime Minister Tony Blair welcomed Afghanistan's leader Hamid Karzai to Downing Street but stopped short of offering more troops.
Mr Karzai has kept up his appeal for a significantly expanded military presence in Afghanistan to help prevent renewed fighting between Afghan warlords.
General Tommy Franks, head of America's Central Command said the overall mission in Afghanistan was far from over.
There has continued to be hard-fought battles between US forces and some remaining Taleban and al-Qaeda strongholds.