Commanders have asked for extra troops for two years, says Mr Mercer
Sending more UK troops to Afghanistan could save lives, Conservative MP Patrick Mercer has said.
His comments came after the prime minister announced plans suggesting a greater role for British troops, during a surprise visit to the country.
Mr Mercer, a former soldier, saw Gordon Brown's announcement as a pledge to send more British personnel.
He said his former regiment was in Afghanistan "and they tell me that the secret to this is extra manpower."
Speaking in Helmand province, Mr Brown pledged greater protection for troops from roadside bombs and better equipment, including more armoured vehicles.
He announced plans for the British to train another 50,000 Afghan troops trained by November 2010, which would enable them to "take more responsibility for their own affairs".
Mr Mercer said: "For at least the last two years, commanders on the ground have been asking for extra troops. That was denied by the government.
Mr Brown said there would be more unmanned surveillance aircraft
"I don't quite know why Gordon Brown only now is announcing this.
"With the extra manpower that is now being promised, perhaps so many lives wouldn't have been lost over the last few months."
The BBC's deputy political editor, James Landale, who was in Helmand with the prime minister, said training that number of Afghans so quickly could require an increase in the number of British troops.
There are currently 9,000 UK troops in the country, mostly in Helmand.
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Mr Brown said another 200 soldiers skilled in countering improvised explosive devices (IEDs) would be deployed in the autumn.
There would also be more unmanned surveillance aircraft, he said.
The latest death in Afghanistan - a Royal Marine killed on foot patrol in Helmand early on Saturday morning - was announced as the prime minister was flying home.
He is the 208th member of the UK forces to have died in Afghanistan since 2001.
Brig Gen Eric Tremblay, spokesman for the International Security Assistance Force (Isaf), said the Royal Marine had "sacrificed his life to help secure Afghans living in the south".
"Like all his fellow Isaf comrades who fell before him in this difficult fight to separate the insurgents from the Afghan population, we shall always remember him."