PRIME Minister Gordon Brown defended Britain's military mission in Afghanistan today, insisting "we cannot, must not and will not walk away".

In a high-profile speech in London, hastily arranged following a series of British deaths in Afghanistan this week, Mr Brown said the military action in the country was our first line of defence against terror attacks at home.

Paying tribute days ahead of Remembrance Day to the 93 British troops who have died in Afghanistan this year, the Prime Minister said: "These men are our heroes today."

Mr Brown acknowledged that Britain's strategy was not without danger or risk.

But he warned that al Qaida terrorists continue to plot terror attacks on Britain from the region, and said: "This mission must not fail."

Mr Brown's speech comes amid polling evidence suggesting that a growing majority of voters now want to see British troops withdraw from Afghanistan within the coming year - with more than half in a survey this week saying they did not believe victory against the Taliban insurgents was possible.

And it follows a call from former Foreign Office minister Kim Howells for the mission in Afghanistan to be halted and the money spent on anti-terror measures in the UK.

In a clear response to Dr Howells' call, Mr Brown said he rejected any suggestion that the commitment in Afghanistan was diverting effort away from security at home.

"Britain has consistently shouldered its fair share of the burden and more - especially in the last three years, since we deployed into Southern Afghanistan, the heartland of the Taliban," he said.

"But when the main terrorist threat facing Britain emanates from Afghanistan and Pakistan; and when, although the sustained pressure in Pakistan, combined with military action in Afghanistan, is having a suppressive effect on al Qaida, we know that they continue to train and plot attacks on Britain from the region ... this mission must not fail.

"It is not easy; the choices are not simple. There is no strategy that is without danger and risk.

"But that is the responsibility of leadership - of government, and of our armed forces. To do what is necessary, however difficult, to keep the British people safe. We cannot, must not and will not walk away."