Brig-Gen Mirwais Noorzai of the Afghan police and Maj-Gen Nick Carter of Nato speak to the press after the Helmand killings

Senior figures in Britain’s ruling Labour party have warned Gordon Brown, the prime minister, of mounting pressure over the war in Afghanistan after five UK soldiers were shot dead by an Afghan policeman at a training camp in Helmand province.

After one of the gravest attacks on British troops since they were first deployed to Helmand province three years ago, Mr Brown said on Wednesday that training of Afghan police remained an “essential element” of the Nato strategy in Afghanistan.

However, Labour politicians warned him that the deaths, which happened as one senior party figure broke ranks with government over the war, could mark a significant new challenge for the UK’s Afghan strategy.

Speaking to the Guardian, Kim Howells, a former foreign office minister, urged Britain to pull back its troops and focus resources on defending the UK border from al-Qaeda.

A former Labour cabinet minister said Mr Howells’s break with policy was significant, not least because he chairs the parliamentary Intelligence and Security Committee.

“What we will start seeing now is a queue of significant political figures on both sides of the Commons preparing to wipe their hands of the whole mission,” said the former cabinet minister. “Frankly, support for that mission across the House has never been weaker than it is today.”

In a week that has also seen Hamid Karzai reappointed Afghan president, there were expressions of concern across parliament. Lord Ashdown, the former United Nations high representative for Bosnia who came near to a similar role in Afghanistan last year, said the west was “quite close to” failure in Afghanistan, an event that would have “baleful consequences”.

Nick Clegg, the Liberal Democrat leader, said the Nato mission was “in trouble because we do not have a legitimate government in Kabul, and we do not have a coherent international plan for Afghanistan”.

Mr Brown told MPs the Taliban rebels may have infiltrated the Afghan police to carry out the attack in Nad-e’Ali district, where the soldiers were training and mentoring police officers. “It appears that they were targeted because they were engaged in what our enemies fear most – they were mentoring and training Afghan forces,” he said.

The killings will stir debate in the US and Europe on how quickly Nato can develop Afghan army capacity to take over responsibility for security from 100,000 foreign troops.

Peter Galbraith, who left as deputy head of the UN mission in Afghanistan amid conflicts over the presidential poll, was not surprised by the killings. “The process of police training and recruiting has been very rushed. There isn’t a lot of vetting of police before they are hired,” he added.