Britain is at serious risk of losing its way in Afghanistan because rising defeatism at home is demoralising the troops on the front line, military commanders have warned.

High-ranking officers, including a former commander of the SAS, have expressed deep concern that the country is in danger of "talking ourselves into a defeat back home" as the war reaches a critical stage.

They say there is "surprise and disappointment" among members of the forces at the constant pessimism in the UK over the conflict, and what looks like a lack of appreciation for what they are achieving at great personal risk and in extremely difficult circumstances.

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Such is the level of concern about the impact of this "negativity" that a number of senior officers have now taken the step of publicly speaking out. They have told The Independent that, in their view, the British people are not getting a true picture of what is going on, and that any loss of public support as a result of this will have highly damaging consequences for the campaign.

There is also anxiety that the Taliban will step up their attacks and attempt to kill more members of the forces in order to create further anxiety at home, fuelling calls for troops to be pulled out.

Lieutenant-General Sir Graeme Lamb, a former SAS commander who has been brought in to play a key role in Nato's new Afghan strategy, said: "We must be wary of talking ourselves into a defeat back home. We hear people saying the fight isn't worth it. Does that mean all the sacrifices which have been made, the deaths and the injuries have been for nothing?

"The troops do not think that is the case – they are stating that belief by what they are doing every day in a dangerous situation. They are the men and women in the arena, and they are certainly not giving up.

"Most of the force are quite stoical; they are just disappointed that their endeavours – what they are trying to do with great courage – are not, it seems, being properly recognised by some back home. It is a pity that they are working so hard to deliver success, not for themselves but the Afghan people, yet there are voices back home who say they are failing.

"To a certain extent, people here understand what is being said is influenced by an election coming up in the not-too-distant future back in the UK. And there have been steady casualties, which is terrible for the families involved. But I don't think those who are so critical have examined what is actually going on here. They have either never come here, or have been on brief flying visits without spending much time in the front line, and that does not give a full picture."

The intervention by the commanders comes at a crucial stage in the conflict, with Barack Obama set to announce the deployment of around 35,000 reinforcements while urging other Nato countries to send up to 10,000 more troops. It also comes at a time of growing opposition to the war in Europe and America – dissatisfaction which was reinforced by the allegations of fraud in the recent election in which President Hamid Karzai retained power.

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Lt-Colonel Matt Bazeley, commanding officer of the 28 Engineer Regiment Group in Helmand, said: "I know why I am here; my soldiers know why they are here, and we think we are right to be here. There is, of course, a degree of nervousness in certain situations, and we have our bad days just like any other profession.

"But the morale among the troops has consistently remained very high. We are extremely grateful for the support from so many people back in the UK. We continue to believe that the man in the Clapham omnibus supports the troops in the front line.

"We can see for ourselves here that progress is being made – more areas are being made secure – and we need to make sure that people back home also realise this, because we are also very aware that there seems to be this pessimism back home, which is rather sad."

Lt-Colonel Jeremy Bennett, commanding officer of 1 Royal Horse Artillery, added: "We are a bit surprised that there is this negativity in the UK about what is going on here, and it is disappointing. I am not sure why that is the case.

"No one is saying the war would be won in the next six months. When this mission began, we knew that British forces would be here for a relatively long time and, there would, unfortunately, be some casualties. But the fact is that the mood among the troops is pretty upbeat. We have inflicted real physical and psychological damage to the Taliban, [and] the Afghan security forces are being trained to take over the defence of their country. We are not saying that we are creating a western European-style state here, but we are trying to create stability and, on the whole, I think we are getting there."

Critics say that failure by the Government to make its policy clear on the Afghan mission has been one of the major contributing factors to the growing public discord. Sir Richard Dearlove, who was head of MI6 when the Afghan invasion was launched, has said that the Government must deliver a strong message to its enemies to avoid the loss of more troops' lives. He said: "The Taliban, if they think we are wavering, will up the pressure and be encouraged to kill more of our soldiers."

General Lamb said: "The next period is going to be hard. The Taliban are not stupid: they see what is being said in Europe and America. There is every possibility that they will raise their levels of attack with Christmas coming up – they know the propaganda value of killing a soldier for Christmas."

Sir Graeme, 56, who worked closely with General David Petraeus in Iraq, is seen by senior American military figures as one of those who was instrumental in turning Sunni nationalist groups (the so-called "Sunni Awakening") against al-Qa'ida, an initiative now seen as a major turning point in the war. On his Afghan appointment, Sir Graeme said that many in the enemy ranks have "done nothing wrong" and there are those in the Taliban rank and file who carry a sense of "anger and grievances which have not been addressed".

The general insisted yesterday: "Things are getting better: the Taliban are suffering badly, but this is not about killing people – it's about establishing security and getting people to talk.

"We do think that the tectonic plates are shifting, but there is a time gap before this becomes apparent. It is always the darkest before daybreak. But give us a bit more time, and we shall begin to see the results by the spring."