The Conservatives have repeated their support for British forces fighting in Afghanistan but expressed anger at not being told about troop numbers.

Shadow defence spokesman Bernard Jenkin began an emergency debate on Thursday about the deployment of 1700 UK combat troops to Afghanistan by reaffirming his party's support for the war on terror.

"There is no doubt about Conservative support for the war in Afghanistan," he said, adding his party stood "shoulder to shoulder" with the government.

"Let there be no doubt that we support the principle behind that decision but it was wrong of the government to make that decision without considering the need for a proper debate."

Jenkin warned against an open-ended commitment which would stretch already heavily committed forces. The government should not "devalue the gold standard" of Britain's military reputation. Uncertainty would result from a split chain of command and split objectives.

He had requested a three-hour debate under Standing Order 24, a parliamentary procedure not used since 1993, after defence secretary Geoff Hoon announced the largest British troop deployment since the Gulf War.

Hoon told the Commons he had already given details of the deployment and that it was vital MPs did not focus on the minor details.

"It is important not to lose sight of the big picture," he said. "Afghanistan is beginning to return to some sort of normality."

"British forces have played a vital role in achieving the successes in Afghanistan."

"Four-five Commando had been held in readiness for precisely this deployment for some time," he said referring to the Royal Marine unit that is being sent to Afghanistan. He did warn that they would be there for "as long as it takes" to deal with the current threat.

Questions also remained over Turkey taking lead of the international security force in the region, Jenkin warned, while only four days of parliamentary time remained before the Easter recess.

A Downing Street spokesman said prior to the debate: "What you are seeing is the green light given to a commitment which was always there in principle from the outset. This is moving to a new phase. What our involvement means is that the military initiative inside Afghanistan will be more flexible."