The main opposition Socialist party said it had voted against continiung the mission as a way to demand "clarifications" on France's strategy in the violence-ridden country.
The Senate, the upper house, will also vote on the issue later.
The decision came as Canada's Globe and Mail newspaper quoted a "secret" Nato report at the weekend saying Taliban fighters who ambushed the 30 French soldiers on August 18 were better armed than their enemy.
The newspaper quoted the report as saying the paratroopers had run out of ammunition after only 90 minutes and had only one radio, that was quickly knocked out, leaving them unable to call for air support.
The mountain ambush east of Kabul was the worst ground attack on international troops since they were sent to Afghanistan in 2001 to oust the Taliban regime there.
Fillon branded the report as "lies and disinformation", saying there had been no loss of radio contact and the troops were "always able to respond" to Taliban firepower.
He said: "We have decided to strengthen our military means in the areas of air mobility, intelligence and support... We have learned the lessons of the murderous ambush."
The prime minister said the extra troops would be deployed in a few weeks.
Nato and the French military also denied the existence of the "secret" report, saying the newspaper was referring to a leaked email sent by an officer to Nato command headquarters in Kabul that gave a partial account of the ambush.
Jean-Louis Georgelin, France's armed forces chief of staff, said that it came from a member of a United States special forces unit that was patrolling with the French soldiers before the ambush.
A Nato source said the officer's email "expressed his personal opinion on what happened during the ambush".
Herve Morin, France's defence minister, said the officer had written up a "fragmented" account immediately after the ambush whose claims about the lack of ammunition proved "completely false".
Morin called for "national unity" ahead of the parliamentary vote later on Monday.
"France must be united behind its troops... in the face of the battle we are fighting against terrorism," he said.
Morin said last week that it was "inconceivable that France, a member of the United Nations security council, the fifth power of the world, would contemplate a retreat".
A poll published after last month's ambush showed 55 per cent of French people supported a pullout from Afghanistan.
Several thousand people took part in about a dozen anti-war protests across France on Saturday, organised by trade unions and left-wing opposition parties.
|Protesters call for the withdrawal of French troops from Afghanistan [AFP]
Critics point to France's involvement in Afghanistan as a worrying sign of French alignment with US policy under Sarkozy, who is considered pro-American compared to Jacques Chirac, his predecessor.
'France go home,' read a front-page headline in English on Monday in the communist daily newspaper L'Humanite, while the Le Parisien paper asked: 'Should we stay?'
The French government faced a no-confidence motion in April over its decision to increase France's involvement in Afghanistan, but that was roundly defeated by the governing party's majority.
About 70,000 international troops, 40,000 of them under Nato command, are helping Afghans fight the Taliban who were ousted from Kabul in a US-led invasion launched after the September 11, 2001 attacks.