The Conservatives are calling for clarity from the government on the role of British troops in Afghanistan.
Troops will operate in the southern region of Afghanistan
Defence Secretary John Reid is being urged to make a Commons statement by the Tories who claim he has failed to give the full facts on the mission.
Documents seen by the BBC suggest the Royal Marines will be expected to find and defeat insurgents in the south of the country.
But Dr Reid said troops are there to focus on peacekeeping.
The Army is preparing to deploy the full task force of 3,300 troops, led by 16 Air Assault Brigade, to start operations in June.
The troops are intended to restore security and stability in the southern region of Helmand, which has seen outbreaks of violence by the Taleban.
But in calling for a Commons statement shadow defence secretary Liam Fox said it was clear MPs have not been given the full facts about the role of British troops.
Briefing documents revealed to the BBC suggest marines will be asked to find and track insurgents in southern Afghanistan, disrupt opposition sanctuaries and eventually defeat them.
Dr Reid has said British forces will focus on peacekeeping and nation building and will not be there to seek out and destroy terrorists - that will be done in a separate operation led by the Americans.
The Ministry of Defence added that these plans have not changed.
UK forces would be involved in counter insurgency to protect themselves and others, not counter terrorism, it said.
But the Conservatives say such definitions may prove difficult to sustain in a hostile area, where British troops have already come under attack.
Two British soldiers were slightly injured last week when a suicide car bomber drove into a military convoy.
The incident happened in Lashka Gar, the Helmand provincial capital, in the south of the country.