Prime Minister Tony Blair is due to be questioned about Britain's role in Iraq and Afghanistan after major changes to deployments in both war-ravaged countries were revealed.
Mr Blair will be grilled by members of the Westminster lobby during his monthly press briefing at Downing Street.
Defence Secretary Des Browne announced plans for Britain to send a further 1,400 troops to southern Afghanistan to help quell the continuing insurgency by Taliban militia.
The deployment, to take place over the summer, will bring the UK's forces in Afghanistan to 7,700 and Mr Browne said that the troops were committed to the mission until 2009.
In an oral statement to the House of Commons, Mr Browne made clear that Britain was having to send more servicemen and women to the troubled central Asian state because of the failure of Nato members like France and Germany to provide troops to fight on the frontline.
Mr Browne lobbied Nato defence ministers to provide more soldiers and equipment for southern Afghanistan at this month's summit in the Spanish city of Seville, where the US promised an additional 3,000 troops, France some close air support and Germany six Tornado reconnaissance jets.
Mr Browne rejected suggestions that last week's announcement of the withdrawal of 1,400 troops from Iraq was dictated by the need to commit more personnel to Afghanistan.
Decisions on the UK's commitment in Iraq were driven by conditions on the ground, he insisted.
Mr Browne acknowledged that the level of missions around the world were putting pressure on the armed forces, but said that both he and the chiefs of staff were confident that the additional commitment to Afghanistan was "manageable".
