Gordon Brown insisted today that the Government would press on with the strategy of training and mentoring Afghan security forces despite the killing of five British soldiers by a rogue Afghan policeman.

The Prime Minister told the Commons that the troops - including five others who were seriously injured in the incident - appeared to have been deliberately targeted by the Taleban, which had claimed responsibility for the attack.

“It appears that they were targeted because they were engaged in what our enemies fear most - they were mentoring and strengthening Afghan forces to make Afghanistan more secure,” he told MPs at a sombre Prime Minister’s Questions.

“While we step up and strengthen our security wherever we can, we will not stop doing what the Afghan Taleban fear most. The sacrifice of our military is great and our resolve must match it.”

He added: "The measure of success in in Afghanistan is that British forces will be able to come when when Afghan forces are able to control the situation for themselves."

Mr Brown described the loss of the five soldiers, who were shot dead at a checkpoint in the Nad-e-Ali district in Helmand province yesterday, as a “terrible and tragic incident”. Six other soldiers were wounded.

While he promised security would be reviewed in the light of the attack, he said it was an essential element of the coalition strategy that British troops continued to work closely alongside the Afghan forces.

“Security will be stepped up where that is necessary, but we cannot desist from the practice which is absolutely essential for the future of Afghanistan and the security of our country and that is training and mentoring the Afghan forces,” he said.

“That means that our troops will be working with the Afghan police and the Afghan army and of course we need to take all the necessary security measures but it is an essential element of the whole coalition strategy that we train up the Afghan forces so that they can take over the security of this country.

“We must not allow ourselves to give up what the Afghan Taleban fear most and that is that we have strong Afghan security force that is Afghan-based to be able to face them.”

The Tory leader David Cameron said it had been an “horrific” incident which underlined the need to “clean up” the Afghan police.

“Clearly the attack raises questions about the infiltration of the Afghan police by criminals, by drug dealers, by militants,” he said.

Earlier, the former Foreign Office minister Kim Howells warned that the shooting of the soldiers had dealt a blow to the heart of the UK’s exit strategy.

“There are many people who have argued that there is only one way out of this for Britain and America and that is to train up the Afghan army and police force so that they can become responsible for their own security,” he told the BBC.

“This is a real blow because it strikes right at the heart of that policy.”

In an article for The Guardian, Dr Howells, who now chairs the intelligence and security committee, called for the phased withdrawal of British troops, arguing that the money would be better spent improved security measures in the UK.

“It is time to ask whether the fight against those who are intent on murdering British citizens might better be served by diverting into the work of the UK Border Agency and our police and intelligence services much of the additional finance and resources swallowed up by the costs of maintaining British forces in Afghanistan,” he said.

“It would be better, in other words, to bring home the great majority of our fighting men and women and concentrate on using the money saved to secure our own borders, gather intelligence on terrorist activities inside Britain, expand our intelligence operations abroad, cooperate with foreign intelligence services and counter the propaganda of those who encourage terrorism.”

Bob Ainsworth, the Defence Secretary, dismissed the claim that Britain could maintain security by relying on internal measures alone.

“We can’t secure ourselves at the borders of Great Britain. If Afghanistan is not secure, then Pakistan will not be secure and Britain will not be secure. That is the whole purpose of us being there,” he told BBC Radio 4’s The World At One.

He acknowledged, however, that action needed to be taken to tackle the problems in the Afghan police. “We absolutely have to get the Afghan security forces where they need to be both in terms and size and ability. That has to involve a cleaning up of the Afghan National Police service,” he said.

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