The chair of the House of Commons intelligence and security committee, Kim Howells, has called for a withdrawal of troops from Afghanistan
A former Labour minister has called for the "great majority" of British troops to be withdrawn from Afghanistan.
Intelligence and security committee chairman Kim Howells said efforts should focus on securing the UK borders against terrorist attack.
Suggestions forces might need to stay in Afghanistan for decades to stabilise the country were "absurd", he said.
But Defence Secretary Bob Ainsworth said the Afghan mission was "inextricably tied" to UK security.
Five UK soldiers died earlier in a single incident in Helmand Province.
Gordon Brown told MPs the Taliban could have infiltrated the Afghan police force to carry out the attack.
During prime minister's questions, he promised an inquiry into the attack, initially blamed on a "rogue" police officer.
And he defended the mission after one Labour MP asked him whether the war against the Taliban could ever be won.
"The sacrifice of our military is great and our resolve must match it," he said.
Speaking before news of the Helmand incident, Mr Howells - a former foreign office minister who supported the war when it began in 2001 - said the opportunity given to the Afghans to tackle the problems blighting the country had "largely been squandered".
A properly planned, phased withdrawal of troops from Helmand province - where the majority of UK forces are based - was necessary, he told the BBC.
"How long do we put up with brave young men and women dying and being injured in Afghanistan?" Mr Howells said.
Public support for the mission had been severely damaged by the recent Afghan presidential election, which was beset by claims of corruption.
Mr Howells said: "I think that part of the problem is the way in which we have handled the Karzai government and the way in which we have handled our NATO allies because the great burden of fighting and of deaths and injuries have been borne by the Americans, by us, by the Canadians and the Danes.
"Very few countries have put their troops in the way of danger as we have and we have borne the struggle against terrorism in Afghanistan hoping that it will reduce terrorism in this country."
BBC defence correspondent Caroline Wyatt said Mr Howells was the first senior politician to call for a withdrawal of UK troops.
Mr Ainsworth told BBC Radio 4's World at One: "If he's saying we can provide for our security at our own borders, then I disagree with him."
He added: "We cannot secure ourselves at the borders of Great Britain... if Afghanistan is not secured, then Pakistan will not be secure and Great Britain will not be secure.
"We are there because this mission is inextricably tied to our own safety in the United Kingdom."
Labour MP Eric Joyce, who resigned as a defence aide in protest at Afghan policy, said Mr Howells was "quite right" that the UK's "main effort" should be on protecting its citizens at home.
While he did not back the immediate withdrawal of UK troops, he said the government must ask itself whether the current strategy was working both in terms of the "blood and money" being spent.
"We need to reappraise exactly what we are doing in Afghanistan."
The deaths of a further five soldiers takes the number of British military personnel killed on operations in Afghanistan since 2001 to 229.
The UK currently has 9,000 troops in Afghanistan and Mr Brown has said he is prepared to send a further 500 troops to Afghanistan if certain conditions are met.
Writing in Wednesday's Guardian, Mr Howells said seven years of military involvement had subdued al-Qaeda's activities in Afghanistan but had not destroyed the organisation or its leader, Osama Bin Laden.
He said the British people were increasingly questioning whether deploying troops was the best way of preventing "Islamic terrorist murders in the UK".
"It would be better... 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."
As chairman of the Intelligence and Security Committee, Mr Howells is responsible for examining the policy and budgets of the domestic and foreign security services, MI5 and MI6.
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