Gordon Brown will today try to bolster crumbling support for the conflict by claiming it is just as vital as the First and Second World Wars.

In a major speech, the Prime Minister will say the troops fighting global terror in Helmand are fighting for freedom like the heroes of the Somme and the Normandy beaches.

Yesterday, a YouGov poll showed support for the Afghanistan war collapsing after five soldiers were shot dead by a rogue policeman.

It revealed 73% of people want our troops withdrawn, and 35% think they should leave immediately, compared with 25% two weeks ago.

But Mr Brown will urge the British public to show the same resolution as the Blitz. He will say: "In the past, we learned of the bravery of British soldiers in the First and Second World Wars in their fight to protect our freedom. So our children will learn of the heroism of today's men and women in Afghanistan - protecting our nation and the world from the threat of global terrorism."

In a hastily planned speech, he will say: "Al-Qaeda, we know, continue to train and plot attacks on Britain from the region. We cannot, must not and will not walk away."

But the PM was urged yesterday to bring troops home by the sister of a soldier killed in the War on Terror. Karla Ellis, 28, speaking at a Q&A with Mr Brown on Key 103 radio in Manchester, said: "How many under-equipped soldiers have to die before the powers that be do the right thing, instead of trying to save face, and bring them home?"

Mr Brown said the strategy was to train up the Afghans to govern themselves. But Ms Ellis, whose brother, Pte Lee Ellis, 23, was killed by a bomb in Iraq in 2006, replied: "I don't think that strategy will work. You can't trust these people."

Meanwhile, the senior UK commander in Afghanistan, Lt Gen Jim Dutton, admitted the five deaths would have a huge impact on British soldiers and warned it was unlikely to be the last incident. Asked if the "atrocity" may cause distrust towards the Afghans, Gen Dutton said: "I think yes it does, initially. Regrettably, I think we have to say it probably won't be the last. But it is a very rare event."

In Afghanistan, three dozen SAS troops have been unleashed to hunt Gulbuddin, the man who shot the five soldiers. A military source said: "The Taliban claim he is safe but I wouldn't rate his chances of staying alive or out of custody beyond a few days. The Brits must catch him."

And another British soldier was killed on foot patrol yesterday morning in Sangin. His death brings the number of Brits killed in Afghanistan since 2001 to 230.

Twelve people were killed and 31 wounded when three gunmen in uniform opened fire at the US Army's largest armoured base in Texas yesterday. One gunman was shot by civilian police and the two others held at Fort Hood.