Extra British troops are to be sent to southern Afghanistan, Defence Secretary Des Browne has confirmed.
The Army expects a major Taleban uprising in the spring
The move, which the BBC understands involves more than 1,000 personnel, comes as about 1,600 troops are being withdrawn from Iraq.
The UK has been reluctant to add to its 5,600-strong force in Afghanistan, as it has reinforced there several times.
The Tories said the move showed British forces were too "overstretched" to carry out duties in both countries.
Mr Browne said an announcement would be made in the Commons on Monday.
But he issued a statement confirming extra troops would be deployed, saying speculation in the media was "likely to be causing concern among our forces and their families".
It is thought that some of the soldiers will come from the Household Cavalry. On Thursday it was announced that the regiment's Blues and Royals unit, in which Prince Harry serves, is being deployed to Iraq.
British forces are in Afghanistan as part of Nato's International Security Assistance Force (Isaf).
Shadow defence secretary Liam Fox said the government had failed to persuade its Nato allies to take their share of the burden in Afghanistan.
"Too many of our European partners are now pocketing the Nato security guarantee, leaving UK taxpayers and the UK military to carry the cost," he said.
"It's clear now that our army's so overstretched that we can't carry out two conflicts."
In his statement, Mr Browne said a decision to send in extra UK troops was taken after efforts to get more help from partners failed.
"We have been trying hard to get other nations to live up to the joint commitment Nato made to Afghanistan and provide more forces, forces which are authorised to fight," he said.
"We will continue to press. But we must be realistic."
Britain has recently revamped its operations in Afghanistan to put most manpower into Helmand province in the south, where the fighting is at its most fierce.
Nato and British commanders have said for some time that more resources are needed if the Taleban are to be defeated.
But until now the government has argued that countries like France and Germany should contribute more.
BBC defence correspondent Paul Wood says commanders on the ground are "screaming for more troops" to deal with the Taleban's expected spring offensive, but Monday's announcement is still likely to be controversial.
He said the governor of Helmand province recently said another 700 Taleban fighters had crossed the border to confront British troops.
"The appeal went out to other Nato nations - such as the Germans up there in the safe part of Afghanistan in the north," our correspondent said.
"Yet it is the British troops once again who are having to reinforce - the third or fourth reinforcement."
The Liberal Democrats said Britain needed to focus on Afghanistan and withdraw troops from Iraq.
Thomas Withington, from the Centre for Defence Studies, explained that the south-west of Afghanistan was proving to be a "stubborn nut to crack".
He told BBC News: "Many answers lie in deploying more troops and having more equipment on the ground but they also lie in securing the border areas.
"And I think what really is required is a two-pronged strategy, to ensure those two things can become a reality."
There are currently about 5,600 British troops in Afghanistan.
The 1,300 of those currently in Kabul will come out of that region shortly.
The majority of those troops will go south to Helmand except for about 400 who will leave Afghanistan.
The remaining 5,200 troops in the country will be bolstered by the expected extra 1,000 troops, making UK troops in Afghanistan 6,200 strong.
LEAD INTERNATIONAL FORCES IN AFGHANISTAN
Locations refer to International Security Assistance Force (ISAF)
Total contributing nations: 37
ISAF total strength: Approx 35,500