He will use a keynote speech to stress that "we cannot, must not and will not walk away," adding that international efforts "will succeed or fail together".
While the Prime Minister's address will not signal any major change in strategy or policy, it will make clear his determination to see through the mission begun eight years ago.
"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-Qaeda – we know that they continue to train and plot attacks on Britain from the region, we cannot, must not and will not walk away," he will say.
Outlining his personal commitment to the cause, and the importance of training Afghan forces, he will tell his audience: "We will not be deterred, dissuaded or diverted from taking whatever measures are necessary to protect our security.
"We will not give up this strategy of mentoring, because it is what distinguishes a liberating army from an army of occupation."
His speech comes as a poll of more than 1,000 Britons revealed opposition to the war had risen sharply in the past fortnight.
Some 35 per cent think all UK troops should be withdrawn immediately, up from a quarter two weeks ago. A further 38 per cent say most British troops should be withdrawn soon, with the rest pulling out within the next year or so, according to the Channel 4 News/YouGov survey.
The debate over Britain's role in Afghanistan has intensified after the murder of five British soldiers in Helmand by a "rogue" police officer on Tuesday.
Former foreign office minister Kim Howells has said Britain would be better off withdrawing its troops and diverting the resources to strengthening internal security.
But General Sir Nick Parker, Britain's most senior commander in Afghanistan, echoed the Prime Minister's defiance when he said any change in military strategy in light of the deaths would be "very damaging".
His comments came as another British soldier was killed in an explosion in southern Afghanistan. The serviceman, from 3rd Battalion The Rifles, died near Sangin yesterday morning, taking the total UK death toll in the conflict to 230.
In Helmand and the UK, the fallout from the attack on the unarmed soldiers inside a secure checkpoint continued.
Tributes were paid to the fallen men, who included a guardsman about to celebrate his 19th birthday and a senior non-commissioned officer who was due to become a father for the fourth time. Colleagues in Afghanistan said the deaths had left them "on edge", as commanders on the ground began reassessing vetting procedures for applicants to join the Afghan National Police (ANP).
British forces were continuing a "huge manhunt" for the gunman, known as Gulbuddin, with surveillance drones, MI6 intelligence officers and special forces liaison officers all involved.
There are suspicions he may have been smuggled out of the area along well-established narcotics-smuggling routes used by the Taleban.
However, the testimony of a police commander in the village of Shin Kalay appeared to undermine suggestions the shootings had been premeditated or orchestrated by the Taleban.
Haji Manan, commander of the checkpoint where the five soldiers were killed, said Gulbuddin, 26, had been acting alone and escaped the scene on foot, dismissing suggestions he had worked with an accomplice and fled on a motorbike.
Mr Manan revealed the rogue officer should have been moved to a different base days before the attack, but troops were too busy fighting the Taleban.
Security sources claim Gulbuddin may have opened fire after getting a "dressing down" from the British soldiers.
In Helmand yesterday, troops from 2 Company, 1st Battalion Coldstream Guards were still training ANP staff. They said that, while the atmosphere had changed, they remained determined to do their job.
Platoon commander Lieutenant Ben Rutt said: "The blokes are perhaps slightly more on edge, more anxious, but they understand that we have to continue with the job. It's crucial we don't tar all the ANP with one rogue brush. A lot of these guys are just trying to do their duty. Unfortunately there's some really rotten apples in the bunch."
In an interview with BBC1's Politics Show to be broadcast on Sunday, Lieutenant General Jim Dutton, deputy commander of international forces in Afghanistan, said the killings would initially lead to greater distrust of Afghan forces but said such incidents were "very rare".
Meanwhile, an expert on the Afghan military operation said the West had to radically scale back its hopes for the country.
Professor Anatol Lieven, from King's College London, said: "The best we can now hope and plan for is a reasonably functioning military state that can hold the north and west of the country, and a few Pashtun cities like Khandar and Jalalabad, and restrict the Taleban to the Pashtun countryside."
Pride and tears for the five who died
CPL NICHOLAS WEBSTER-SMITH
THE 24-year-old, main image was described by his family as "one of the most loving, generous, kind-hearted men you could meet".
Friends of the Royal Military Police corporal, from Saundersfoot in Pembrokeshire, left postings on his Facebook page expressing their shock and disbelief. A series of poignant messages between him and his girlfriend, Emma Robinson, before he died were also on the site.
During his short military career, he undertook operational tours to Afghanistan and Kosovo, as well as volunteering for a six-month deployment to the Falkland Islands.
WARRANT OFFICER CLASS 1 DARREN CHANT
The 40-year-old who was due to become a father for the fourth time, died on the day he was to be told he had won a commission as an officer.
The London-born Grenadier Guardsman carried a colleague whose leg was blown off in a bomb attack more than a mile to safety earlier this year.
He was reportedly on a shortlist of three to become Academy Sergeant Major at Sandhurst. His wife Nausheen, said she was "immensely proud" to say he was her husband.
GUARDSMAN JIMMY MAJOR
The Grimsby soldier had been due to turn 19 next Thursday and his father, Adrian, said they had already posted presents to Afghanistan for him.
Asked if the many tributes left in the garden of the family home helped with dealing with his son's death, he said: "They do because it means people are thinking of him." A Union flag has been fluttering on the side of the family house since the day his son had gone to Afghanistan.
CORPORAL STEVEN BOOTE
The 22-year-old was a Territorial Army soldier from Birkenhead who had volunteered to deploy to Afghanistan. He worked as a security team leader at a Tesco store and had hoped to join the police when he came back to Britain.
The motorbike enthusiast's parents, Margaret and Anthony, said he would "light up a room with a single smile", adding: "Steven, we are all so proud of you and you will always be our hero."
Cpl Boote's company commander, Major Phil Hacker, said:
"He inspired confidence in all those he served with and we are all so proud and feel so humble to have served with him."
SERGEANT MATTHEW TELFORD
The father of two and Grenadier Guardsman, described as a "gentle giant", joined the army at 16, and was 37 when he died.
His widow, Kerry, said she did not know how to explain his death to their four-year-old son, Harry. "I don't want to say that nasty men have killed daddy – I want to be able to tell him that he's in heaven now and that he's gone to be with the angels," Mrs Telford said.
'You were the best ever, Tam – brave soldier, son, good friend'
THE mother of a soldier who died weeks after being wounded in Afghanistan yesterday remembered her "brave" son as "the best there ever was".
Corporal Thomas Mason, 27, known as Tam, was caught by an improvised explosive device in Kandahar province on 15 September.
Cpl Mason, of the Black Watch, 3rd Battalion The Royal Regiment of Scotland, died in Selly Oak Hospital in Birmingham on 25 October.
More than 300 mourners packed into Trinity Parish Church in Cowdenbeath for his funeral yesterday.
During the service, the Rev Nicola Trail read a tribute written by his mother, Linda Buchanan.
In it, she spoke of her unending love for her son and said: "You are the best there ever was, Tam – brave soldier, son, good friend.
"I know in time the pain we feel from losing you will end. So we will struggle by without you, without your laugh, your smile.
"I know we'll meet again, Tam, even if it takes a while."
Mrs Buchanan was joined at the service by Cpl Mason's 21-year-old widow, Kylie, and a large number of family members and colleagues.
Meanwhile, a memorial to another Black Watch soldier killed in Afghanistan was unveiled yesterday at his former high school.
Private Kevin Elliott, 24, died in a rocket-propelled grenade attack in Helmand Province in August.
His family and friends were joined for the occasion by staff and pupils at Braeview Academy in Dundee. Major Ronnie Proctor of the Black Watch Association and Dundee's lord provost, John Letford, also attended the ceremony to pay their respects.
In Wootton Basset, the body of an explosives expert killed while defusing a bomb in Afghanistan was repatriated.
Christina Schmid, the widow of Staff Sergeant Olaf Schmid, said he was a hero and a "preserver of life".
"He was a very brave man, there was no other man above him, I am so proud of him," she said.