The prime minister is facing fresh questions over his policy in Afghanistan following the resignation of a parliamentary private secretary.
In his resignation letter Eric Joyce, often described as an "ultra-loyalist", warned Gordon Brown that he was failing to make the case for the continuing intervention.
With British casualties continuing to rise, the MP warned that the public will not "accept for much longer that our losses can be justified by simply referring to the risk of greater terrorism on our streets".
"Nor do I think we can continue with the present level of uncertainty about the future of our deployment in Afghanistan," he added.
Joyce also attacked government ministers for briefing against Army chiefs who have questioned whether troops in Afghanistan are properly equipped.
"Behind the hand attacks by any Labour figure on senior service personnel are now, to the public, indistinguishable from attacks on the services themselves," he warned.
Joyce also said that Labour would lose the next election unless it "got to grips" with defence.
"I believe the next election is ours to win, thanks greatly to your personal great economic success," he wrote in his letter to the prime minister.
"But we cannot win unless we grip defence. Above all, Labour must remember that service folk and their families are our people.
"We say that we honour them for their risk, bravery and sacrifice and we must at literally all costs continue to show by our actions that we mean it."
Joyce attacked Britain's NATO allies for not pulling their weight in Afghanistan.
"We also need a greater geopolitical return from the United States for our efforts," he told the prime minister.
"For many, Britain fights; Germany pays, France calculates; Italy avoids. If the United States values each of these approaches equally, they will end up shouldering the burden by themselves."
Joyce, one of a small number of MPs to have served in the armed forces, was parliamentary private secretary to defence secretary Bob Ainsworth.
Shadow defence secretary Liam Fox said the letter showed the Labour administration "has now become the government of fear and smear".
Liberal Democrat leader Nick Clegg said: "Eric Joyce confirms what I have been saying for a long time, our approach in Afghanistan is over-ambitious and under-resourced."
In his response to the resignation letter, the prime minister stressed "the importance of the effort in Afghanistan to defeating terrorism, and of supporting, at all times, our armed forces".
"I am determined that nothing will distract my government from this most vital of tasks," he said.
Initially a private in the Black Watch, Joyce attended the Royal Military Academy at Sandhurst and was commissioned into the Adjutant General's Corps in 1987.
He served in the UK, Northern Ireland, Germany and Central America and left the army in 1999 to serve as the public affairs officer at the Commission for Racial Equality (Scotland).
He was elected as MP for Falkirk West in the by-election of December 2000.
It is understood he told Ainsworth he wanted to resign more than three weeks ago and the defence secretary tried to change his mind.
Some Labour MPs have publicly backed Joyce’s decision.
Former minister Peter Kilfoyle told the BBC: "This reflects the growing concern both here and in the US about why we are still in Afghanistan and with the changing objectives. It is never quite clear what we hope to achieve there."
The UK has deployed 9,000 troops to Afghanistan and more than 210 British troops have died in action, with many more wounded.
4 September 2009