Gordon Brown is expected to announce that Britain is to send an extra 500 military personnel to Afghanistan.
The UK has about 9,000 soldiers in the country, the second-largest contingent after the US, but there have been calls for increases in Nato troop levels.
To date, there have been 221 deaths among UK forces there and the PM has faced some calls to bring troops home.
Mr Brown will address MPs at 1230 BST, after the first prime minister's questions of the parliamentary session.
He will begin his statement with a sombre tribute to British forces, reading out the names of the 37 servicemen who have died in Afghanistan since the last prime minister's questions in mid-July.
It is believed that Mr Brown will agree to the deployment of 500 more British troops but with some caveats.
The BBC's defence correspondent Caroline Wyatt said: "The prime minister will want assurances from military chiefs that the extra troops will be properly equipped.
"He'll also expect Britain's NATO partners to follow suit by offering more forces themselves."
British forces have been in Afghanistan since October 2001. More than two-thirds are stationed in the southern province of Helmand, a Taliban stronghold.
Last week Mr Brown's spokesman said: "We have always said that more troops would have to be subject to a number of criteria - the feasibility of sending the right equipment, the right strategy internationally, and particularly this issue of 'Afghanisation' which the president and prime minister talked again about yesterday, and obviously proper burden-sharing."
In his speech to the Conservative Party conference, leader David Cameron pledged to send more troops to Afghanistan to speed up the training of Afghan soldiers.
The Liberal Democrats have claimed the Afghan mission is "failing" and called for a new strategy and "a political surge" rather than more troops.