The British public is losing faith in the military mission in Afghanistan and politicians must do more to win back popular support, the head of the British Army warns today. In a blunt assessment, General Sir David Richards, the Chief of the General Staff, said that gaining popular backing for the intervention has been a struggle and warned: We need to do better. In a letter to the media , Sir David also cautioned politicians against seeking a premature exit from Afghanistan, saying that leaving the country too early would put Britain at risk of terrorism and destabilise south-east Asia. We should not allow our security policy to be driven by opinion polls, he said. Britain has 9,000 troops in Afghanistan and Gordon Brown has authorised the deployment of another 500. The US is also considering a big reinforcement. Yet polls show voters on both sides of the Atlantic are turning against the mission. The generals warning came as the Afghan deployment came under fresh criticism. Nick Clegg, the Liberal Democrat leader said that without a significant change in strategy, the mission is condemned to failure. And Sir Christopher Meyer, a former British ambassador to Washington called the intervention a waste of blood and treasure. Insisting that Britains involvement is essential for national security, Sir David painted a stark picture of the costs of failure in Afghanistan. It is not a coincidence that, since the mission began, al Qaeda has been unable to plan or orchestrate any further atrocities against the west from within Afghanistans borders, he said. Nor should we overlook the impact of failure on the stability of the region. In particular, there would be a severe risk to the security of nuclear-armed Pakistan. In an appeal to the Prime Minister and other politicians, the general urged greater efforts to sell the war to the public. It has been a struggle to persuade the British public about all of this and we need to do better, he said. Eight years after British troops first entered Afghanistan to topple the Taliban regime in Kabul, there is no clear timetable for an end to the Western mission in the country. Taliban insurgents control large parts of country, the drug trade is rife and the economy remains one of the poorest on earth. A total of 221 British service personnel have died in Afghanistan since 2001. Thirty-seven servicemen have been killed since July in bloody summer that has strained public support. Ministers have also faced persistent questions about the equipment and support given to troops on the frontline. Commanders have said that if more helicopters were available, some casualties could be avoided. A Daily Telegraph/YouGov poll in August showed 62 per cent of people opposed British troops staying in Afghanistan. Another YouGov survey last week showed that only 27 per cent of voters support a long-term deployment. US voters are also cooling on the Afghan mission, and British commanders privately predict that President Barack Obama will seek to reduce the US presence in Afghanistan before the next US presidential election campaign in 2012. Mr Brown has been signalling moves towards an eventual end to the British mission. Last week, he told MPs he wants to draw up benchmarks and timelines to chart the handover of security operations to Afghan control. The Conservatives have also hinted at seeking a way out of Afghanistan. Liam Fox, the shadow defence secretary, on Sunday said that the West must limit its ambitions for Afghan democracy. He said: We've got to stop judging Afghanistan by Western standards. We're not trying to apply a Jeffersonian democracy to a 13th century state. If we try that, we'll be unsuccessful. Earlier this year Sir David, sparked political controversy by suggesting that Britain may have to support Afghanistan for another 40 years to deliver stability in the country. In his letter -- a response to Daily Telegraph columnist Jeff Randall's call for withdrawal -- the general made clear that British troops leave long before that. Although it will take a few years to reach that point I am quite clear that the campaign is winnable and that we and our Afghan and international partners have the strategy and resolve to see it through, he said. But even when adequate security levels have been achieved, Afghanistan, one of the poorest and least developed countries in the world, is likely to need some form of international help for far longer. Sir Davids appeal for continued backing for the war came as the Lib Dems moved closer to calling for the withdrawal of British troops from Afghanistan. In a BBC interview, Mr Clegg said that support for the war could not be unconditional" and suggested the Nato mission is failing. "The present strategy is failing so it needs to be changed and the discussions which are taking place in Washington at the moment are immensely important in working out whether we have got a strategy which will succeed. "If that strategy, if that new strategy is, in our judgment, the wrong strategy, which will condemn our soldiers to failure, then of course we will revisit our support, of course." He added: "I think if we carry on, on the present course, we are almost certainly condemned to failure. I want us to succeed in Afghanistan." Sir Christopher, who was British ambassador to the US from 1997 to 2003, wrote in a new book that the Afghan mission lacks any clear aims. He said: If this madcap venture is to take 40 years, as General Sir David Richards, chief of the general staff, averred this year, no conceivable national interest can be served by such an eccentric concentration of resources on a country of marginal importance. (Daily Telegraph)