Despite fresh warnings of "overstretch" in the Army, UK numbers in the war-ravaged country are to hit a record high of 8,030 by next spring.

Announcing details of the plans to ministers, Defence Secretary Des Browne said the extra troops would improve security for British servicemen and women. They will also help train Afghan security forces and assist with reconstruction and development.

Mr Browne told MPs that 400 British posts in Afghanistan are to be withdrawn, while 630 new ones are to be created.

The new deployments include personnel to man reconnaissance and warning systems, additional helicopter crews to protect Kandahar airfield and the replacement of the RAF's Harrier force with Tornados.

Mr Browne said the tactics of the Taliban had shifted from insurgency to terrorism, with the increasing use of mines, roadside bombs and suicide vests.

But Prime Minister Gordon Brown signalled there would be no corresponding troop reduction in Iraq as a "trade" for the additional 230 personnel earmarked for Afghanistan.

Earlier, at a press conference with the Prime Minister, visiting US President George Bush warned that the insurgents had to be tackled "or Afghanistan would come to us".

On Iraq, the Prime Minister said UK troops were moving from a combat to an "overwatch" role, but added that he would not impose an "artificial timetable" on withdrawal.

Mr Bush also sought to play down persistent rumours of a rift between the two men over the UK's plans for drawing down troops in Iraq, lavishing praise on Mr Brown for his stance.