Gordon Brown and three cabinet ministers have moved to shore up wavering support among Labour backbenchers for the war in Afghanistan.
In a joint email to the party's MPs, they insist the military action is essential for Britain's security and argue UK forces are receiving "the best possible support".
The moves comes after 23 Labour MPs called for an urgent Commons debate and vote on Britain's involvement in Afghanistan, which is now in its ninth year.
The party's MPs have reported a sharp increase in numbers of constituents opposing the war as the British death toll in Afghanistan mounts.
Mr Brown was joined by Foreign Secretary David Miliband, Defence Secretary Bob Ainsworth and International Development Secretary Douglas Alexander in the attempt to steel their MPs' nerve. They have also circulated a draft letter for MPs to send to constituents who lobby for troops to be pulled out of the war zone.
The email message to MPs, obtained by The Independent, says: "The reason Britain has sent military forces to Afghanistan is clear – our national security is at stake. Afghanistan was where al-Qa'ida trained and planned terror attacks – including September 11th."
It insists the UK is not operating alone, arguing that the strategy in Afghanistan is supported by the United Nations as well as "43 troop-deploying countries, and many others who contribute funding and civilian staff".
MPs are told that Britain's presence has increased from 5,500 troops in the autumn of 2006 to around 9,000 today. Another 500 that have been promised will not be deployed unless their equipment is satisfactory, other nations agree to send more soldiers and the "new Afghan government steps up to the challenges it faces, including corruption." The letter adds: "Above all, we would never commit British troops to Afghanistan unless we were convinced that our security demanded it."
In response to fears over the equipment supplied to the forces, the ministers say the Government has spent more than £1.2bn on new vehicles for Afghanistan in the past two years and increased helicopter numbers in Afghanistan by more than 60 per cent between 2006 and 2009.
The ministers say "stable and legitimate Afghan government leadership is key" to their approach. They admit they were "dissatisfied by the level of intimidation and fraud" in this summer's election, but add: "It is still important to remember that despite the Taliban's intimidation, millions of Afghans voted."
Mr Brown has delivered three speeches since September in which he justified Britain's involvement in Afghanistan. Mr Miliband, who was in Afghanistan yesterday for the inauguration of President Hamid Karzai, also sought this week to soothe fears that troops could be bogged down indefinitely fighting the insurgency. However, a motion demanding a Commons debate on "the UK's role in Afghanistan, the objectives of this intervention and the timescale over which the Government believes these goals will be achieved" has been backed by 23 Labour MPs, including four former ministers.
The motion's sponsor, Frank Field, told The Independent: "We don't want an email. We want a debate. There's not a member that doesn't grow more disturbed by the scenario unfolding before us in Afghanistan. Even if some of them do not wish to change policy, there's widespread and deep unease among the Parliamentary Labour Party."
Harry Cohen, the MP for Leyton and Wanstead, was scathing about the contents of the email, claiming it was full of inaccuracies and omissions. He said: "All those troops and all those civilians who have died – they are neglected in this letter." He added: "Before the election they were saying our principal reason for being in Afghanistan was to bring democracy – where is that in this letter?"
Mr Cohen also questioned the claim that the war was in Britain's security interests: "The Iraq war increased the risk of terrorism in this country. Rather than improving our safety, being in Afghanistan could make it worse."
A senior government source said: "We want to get the message out to everyone – not just in the country but to colleagues on the back benches.
"They are worried and concerned about this issue and they want to see strong leadership from Gordon – and he is giving it."
In the Lords, Lord Guthrie of Craigiebank, a former chief of the defence staff, repeated a call for an end to government "dithering" over Afghanistan but said the situation was not without hope. "Our servicemen and women in the front line need to know the Government and the people are for them and the Government and the people are resolute. They do not want to see – and I use a word others have used – dithering."
In defence of war: Brown's email
* "The reason Britain has sent military forces to Afghanistan is clear – our national security is at stake. Afghanistan was where al-Qa'ida trained and planned terror attacks – including September 11."
* "Our strategy in Afghanistan is supported by the United Nations, 43 troop-deploying countries, and many others who contribute funding and civilian staff. We do not operate alone."
* "The training of Afghan forces is central to our mission. Success will be when the Afghan National Army and National Police are strong enough to provide for their own security."
* "Our strategy is combined with strong support for Pakistan in its efforts against terrorism and extremism on the other side of the Afghan-Pakistan border... "If the coalition withdraws from Afghanistan prematurely, it could have enormous consequences for Pakistan.
* "We have increased our military forces in Afghanistan progressively from 5,500 in the autumn of 2006... to around 9,000.
* "On 14 October we announced our intention to increase this further to 9,500 subject to certain conditions being met... We will not implement this increase unless we are sure the balance of risk taken on equipment is judged by the military to be acceptable – and unless there is an agreed strategy with fair burden-sharing across the international coalition and unless the new Afghan government steps up to the challenges it faces, including corruption.
* "Above all, we would never commit British troops to Afghanistan unless we were convinced that our security demanded it."
* "We are committed to ensuring our forces in Afghanistan have the best possible support. Military spending on Afghanistan – from the Reserve, on top of the defence budget – is now at £390,000 per soldier fighting, compared to £180,000 in 2006."