Mr Reid admitted the troops - who will be on a mission to protect civilians and deter terrorists - would face risks, but told the Commons the bigger than anticipated deployment was needed to help curb the huge narcotics industry and provide security.
The UK's military presence in Afghanistan, which currently stands at around 850 troops, will increase dramatically, with around 3,300 extra combat troops being sent to the lawless southern province of Helmand by July.
A forward team of around 850 engineers will begin building an encampment for the new task force in Lashkar Gar, the capital of Helmand, next month. The area is dominated by the opium trade.
The US troops currently stationed in the south, who are focusing on purely counter-terrorism operations - have been targeted by suicide bombers.
Mr Reid said the "potent" UK force, which will include attack helicopters, would briefly peak at 5,700, falling to less than 4,700 when the engineers had finished their work.
He said it was vital "we put the right forces in to do the job and do it safely and well", and told MPs the risks faced by troops were "as compared to the dangers to our country and our people of allowing Afghanistan to fall back into the clutches of the Taliban and the terrorists."
The mission, which will last for two years, would cost £1bn over five years, he said.
In 2004, the UK agreed to take command of the Nato International Security Assistance Force (Isaf) in Afghanistan this May. The British general Sir David Richards will establish a command post in the capital, Kabul.
Britain will also be in command of a forward support base in the southern city of Kandahar.
The British task force will include troops from the 16th Air Assault Brigade and an airborne infantry battle group, including eight Apache attack helicopters.
A small number of reservists, mostly drawn from the Royal Rifle Volunteers and the Parachute regiment, will also be involved.
Nato, which took over Isaf from the US in 2003, has until now been patrolling the relatively peaceful north of the country, but is expanding its operations southwards.
There are currently 9,500 Nato troops in the country but this number will grow to more than 16,000 as the alliance takes over security control for three quarters of Afghanistan throughout the year.
Britain and Canada will contribute the most troops to the increase in Nato numbers. UK ministers are awaiting the results of a vote in the Dutch parliament on February 2 which will decide whether 1,200 Dutch troops will also be committed.
Mr Reid said Australia and New Zealand could also make new commitments of troops to Afghanistan.
He stressed the need to help rebuild the country and create a "real alternative" to opium harvesting, which provides 90% of the heroin on Britain's streets.
The Tory defence spokesman, Dr Liam Fox, said the Nato mission embraced "noble ideas" which his party supported, but warned it would hold the government to account for any failures in policy.
Some aid organisations said it was important that the British troops should not interfere with humanitarian work.
"There are high expectations of the British military to avoid the past mistakes of US and other troops, which have to some extent fuelled insecurity," a statement from Care International said.
