Tony Blair defended the deployment of a 1,700-strong Royal Marine corps to Afghanistan as the Conservatives sought assurances that the force would not get "bogged down".

Mr Blair, speaking at prime minister's questions in the Commons, said it was essential to get the job of mopping up the remaining al-Qaeda and Taliban fighters "done fully".

He denied there was any conflict between having one British force to keep the peace and another in a combat role. "I don't believe there is any mismatch between the security forces operating in Kabul and those forces that are going to deal with the last elements of the al-Qaeda and Taliban because that is a necessary part of prosecuting the war in Afghanistan to a successful conclusion."

Mr Blair spoke ahead of a three-hour emergency debate prompted by the Tories. In the debate, Bernard Jenkin, shadow defence secretary, supported "the principle" of the new deployment but sought more information about the marines' role.

He asked if intelligence was available to assess the number and capabilities of the forces British troops would come up against. "How will we avoid being bogged down in such fighting?" he asked.

Of the peacekeeping role in Kabul, he said there must be a clear understanding that this was separate to the combat role.

Michael Portillo, former Conservative defence secretary, supported the deployment. "This is not an American war, it is a war of many allies and it is a war in which we committed ourselves to defeat terrorism . ..If we failed to take this decision then the terrorists would not lose much time in regrouping."

Mr Jenkin challenged Geoff Hoon to explain his comment that it was an "open-ended" commitment. However, the defence secretary did not put a date on the troops' return. "This decision will be taken in the light of the circumstances on the ground and in the light of the tasks these troops may undertake. Our exit strategy is that we will leave when the task is completed."

The deployment was not prompted because the US forces "needed to be rescued or had somehow failed. They did not; they fought and they won against a heavily-armed enemy dug into prepared defensive positions in the rocks and caves."

It would not overstretch the armed forces. While they were "extremely busy", the forces involved had been on standby to go into Afghanistan for several months.

Asked how the operations would avoid the "bloody nose" suffered by Russians in Afghanistan in the 1980s, Mr Hoon said he had discussed this with his Russian counterpart yesterday.

"Very largely they tried to occupy ground and that obviously made them vulnerable to attack . . . the whole purpose of the operation we are discussing is to ensure that in swift search-and-strike operations we remove any threat not specifically to our own forces . . . but more generally to the stability of Afghanistan."

Former Labour defence minister Peter Kilfoyle asked sif any of the operations compared with "search-and-destroy" missions in the early stages of the Vietnam war. Mr Hoon said: "Not in the least - very different terrain, very different circumstances and a very different history." Editorial Comment, Page 20 Personal View, Page 21 www.ft.com/terror

Britain 'has no plans for missile defence systems'

Geoff Hoon has said that Britain has no plans to acquire missile defence systems because it sees no near-term threat of ballistic missile attack on the UK or on British troops abroad, writes Alexander Nicoll.

The defence secretary yesterday told the all-party Commons defence committee that the government was concerned about efforts by North Korea, Iran, Iraq, Libya and Syria to develop weapons of mass destruction and the means to deliver them.

"We are not taking this potential threat at all lightly," he said. However, there was no evidence that any country that might develop the capability had any intention to target the UK. Although terrorist groups had tried to acquire such weapons, there was no evidence they had succeeded.