A substantial number of extra British troops are to be sent to Afghanistan, the BBC has learned.

Defence Secretary Des Browne will give details of the new deployment to the House of Commons on Monday.

The UK has been reluctant to add to its 6,000-strong force there as it has reinforced several times already.

Shadow defence secretary Liam Fox said the move showed that British forces were too "overstretched" to carry out duties in both Iraq and Afghanistan.

The move comes as the government announced that about 1,600 troops would be withdrawn from Iraq.

British forces are in Afghanistan as part of Nato's International Security Assistance Force (Isaf).

Revamped operations

Mr 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."

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.

According to a report in the Guardian newspaper, Mr Browne will announce an extra 1,000 troops, but that figure has not been confirmed.

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 Radio 4's Today programme: "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."