The UK is expected to send 3,500 extra troops to Afghanistan in April or May, bringing the total number there to more than 4,000, the BBC has learned.
UK troops are operating in areas run by warlords
Defence Secretary John Reid is to address MPs on the matter on Thursday.
The UK takes control of Nato forces in Afghanistan in May, with soldiers due to oversee reconstruction efforts.
Most of the troops are set to go to the volatile area of Helmand in the south, and are likely to face a number of insurgent groups, correspondents say.
It is thought that 16 Air Assault Brigade will be involved because they are currently involved in a major training exercise in the UK in preparation.
There are currently about 850 British troops operating in Afghanistan.
The final decision on numbers will be made by the Cabinet on Thursday.
Mr Reid would not confirm the numbers before then, but Nato Secretary General Jaap de Hoop Scheffer told the BBC the 3,500 figure would be "not far off the mark".
BBC defence correspondent Paul Wood said the deployment had been described as the most ambitious and challenging mission yet undertaken by Nato.
"British officials stress that the Nato troops will not be carrying out a US-style counter-terrorist operation, but they will defend themselves if attacked," he said.
The Taleban, adopting suicide bombings as a means of attack, is very active in Helmand province, which is also a big opium-growing region.
After taking control of the Nato force in May, the UK is due to expand operations into the south - currently patrolled by the US - later in the year.
But in the Commons on Wednesday, Mr Reid said newspaper reports, suggesting 4,000 troops would go, could "be right... or very, very wrong".
Mr Reid told MPs that before a decision was made, British military strength had to be sufficient to achieve its aims and aid to Afghan farmers enough to offer an alternative to opium.
The minister said he was satisfied over these two aspects but not about the "Nato configuration of military troops around us".
He was speaking in response to an Urgent Question from shadow defence secretary Liam Fox.
Mr Fox told the BBC: "What we want to know is, if British troops are being sent there - and they are going to one of the toughest parts of Afghanistan - is everything possible being done to guarantee they can operate with as much safety as possible?"
Liberal Democrat peer Lord Gardam told the BBC he believed it would be difficult for Mr Reid to give firm numbers until he knew what the UK's Nato allies - especially the Dutch - were doing.
Nato's International Security Assistance Force mission currently numbers about 9,200 troops. It is expected to increase the overall number to about 15,000.
Next week, there will be a conference in London on the future development of Afghanistan.