The killing overshadowed Cameron's previously unannounced visit, curtailing his plans for security talks with political leaders in Lashkar Gah, the provincial capital of Afghanistan's restive Helmand province.
Britain's defense ministry confirmed the soldier, who was reported missing in the early hours of Monday from a base in central Helmand, had been found shot dead following a huge search effort across the province.
Lt. Col. Tim Purbrick, a spokesman for British troops in Helmand, said the soldier had been discovered in the Nahr-e-Saraj district of Helmand.
"The soldier was reported missing from his checkpoint very early this morning and an extensive search of the areas was conducted throughout the day to locate him," Purbrick said.
He said the soldier had suffered gunshot wounds, but that the exact cause of death is still to be established and an inquiry was being carried out.
Cameron's aides said he had been deeply saddened by news of the soldier's killing.
Earlier, Taliban spokesman Qari Yousef Ahmadi claimed insurgents captured the soldier during a firefight with NATO troops in the Babaji area of Helmand province and that he died in the crossfire. Ahmadi said that six other NATO troops died in the gun fight.
The Taliban claim could not be independently confirmed, and NATO said the coalition had no reports of a gun battle in Babaji on Monday.
British officials did not release the soldier's name, but said his family has been notified. Only one soldier from the NATO-led force is presently believed to be being held in captivity. Bowe Bergdahl, a 25-year-old U.S. Army sergeant from Hailey, Idaho, was taken prisoner June 30, 2009, in Afghanistan. He is believed held in Pakistan.
Cameron is making a two-day visit to Afghanistan to discuss plans for a modest drawdown of Britain's 9,500 forces - mainly based in Helmand - ahead of a formal announcement to Parliament on Wednesday.
Helmand's provincial capital, Lashkar Gah, is scheduled to transition security operations to Afghan control later this month - though the region remains a particularly violent insurgent stronghold.
Before the soldier's death had been confirmed, Cameron told reporters that his disappearance had underscored the difficulties ahead as Afghan forces take on security responsibilities.
"Of course we are going to have challenges and problems right up until the end of the mission. Of course, it is a very regrettable incident," he told reporters at Camp Bastion, a British base in central Helmand.
The frantic search for the missing soldier saw Cameron ditch plans to visit a smaller base in Lashkar Gah and meet Helmand's governor at his residence.
"I just said when I got here, `Don't bother about flying me around Helmand province, just use all you have got to try and find that person,"' Cameron said.
However, the British leader insisted that - despite the setback - the overall security picture in the region was improving, and that Afghan forces were growing in capability.
"We are entering a new phase in this country," Cameron told around 200 U.S. Marines as he addressed a Fourth of July celebration at the U.S. Camp Leatherneck. "As President Obama said last week, and as I say today, it is right that we are going to be able to bring some of our troops home, as the Afghans become more confident about delivering their own security."
He said international forces were "not here to create a perfect democracy, we're not here to create a perfect country - but God knows, we're doing some good things along the way - we really have to make sure that this country can try to look after its own security."
Cameron confirmed he will announce to lawmakers the detail of a minor withdrawal of U.K. forces - likely to see about 500 of the country's 9,500 troops return home. An additional 450 personnel deployed on a temporary mission are already being pulled back by February.
"There will be an opportunity to bring some British soldiers home, we are talking relatively small numbers and over a period of time," he said.
He said that British troop numbers will likely stay the same through next summer, but insisted that the 2014 international deadline of quitting a combat role in Afghanistan was not negotiable - even if local Afghan forces struggle to build on tentative signs that they are capable of providing the nation's security.
Gen. David Richards, the head of Britain's military, said the international strategy should no longer be to attempt to defeat Afghanistan's insurgency by the end of 2014, but to ensure that local troops and soldiers were capable of performing the task themselves.
"We possibly set expectations at an unrealistic level, and tried to do too many things," Cameron said. He added that work on development projects were "all very good things, but the first priority is security and that is the mission critical part of why we are here."