It says their total number will be increased to 5,700.
Australia may also join their push into southern Afghanistan, along with Denmark, said Britain’s Defence Secretary John Reid.
The 4,600 additional British troops include 3,300 for a special task force charged with reconstruction and fighting the drug trade.
The task force will be based in the volatile southern Helmand province, where members of the ousted Taliban regime still lurk.
The airborne assault and infantry troops will be armed with eight new US-made Apache and four Lynx attack helicopters as well as six Chinook transport helicopters.
More than 1,000 troops will also be sent to the Kabul headquarters of the British-led Allied Rapid Reaction Corps (ARCC), which assumes command of the North Atlantic Treaty Organisation (NAT0) force from May until February 2007.
Following the peak in the middle of the year, Mr Reid explained troop numbers were then expected to stabilise at about 4,700, including the 1,100 British troops already there.
The contingent will form part of a three-year NATO expansion to some 18,500 troops, including 9,000 in the south, including soldiers from the United States, Canada, Romania and Estonia.
It is the third phase of the expansion of the NATO force, in a bid to stabilise and rebuild the nation and help impose the authority of Afghan President Hamid Karzai's government.
Separately, nearly 20,000 US troops are deployed in Afghanistan, including in southern and eastern Afghanistan to hunt for Taliban remnants and their al-Qaeda allies.
Mr Reid acknowledged the forces faced risks in parts of Afghanistan where the Taliban remained active and the influence of drug traffickers was strong.
But he added: "Those risks are as nothing 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 announcement of the STG1 billion ($A2.38 billion) deployment followed "a unanimous decision" by Prime Minister Tony Blair's cabinet, said Mr Blair's official spokesman.
The deployment will not lead to a reduction of Britain's military presence in southern Iraq, said Mr Reid.
The US-led force and ISAF -- which currently has 11,000 soldiers from 37 countries -- have had separate roles in Afghanistan in the years since the ouster of the Taliban by US led forces in 2001.
US military officials have not said how many of their 18,000 troops, currently deployed in Afghanistan as part of Operation Enduring Freedom, would come home as a result of the new NATO deployments.
The United States and Canada already have troops in the south and Mr Reid expressed hope that more countries, including the Netherlands would join.
"We're optimistic that the Dutch... will also apply deployment forces," he said, stressing that the overall aim of the operation was not to "make war" but "to protect and deter".