A defence ministry spokeswoman said the death occurred during an "incident" on Sunday in Helmand province in the southwest of the country. She gave no further details.
"It is with regret that we believe UK forces have suffered a fatality as a result of an incident in Helmand province, Afghanistan," she said.
"This is very recent information. I have no details of what happened."
The death comes as the insurgency by Taliban rebels is at its worst since US-backed forces overthrew the Islamist government in 2001.
The Afghan government meanwhile is considering creating units of armed tribesmen to protect parts of the country from intimidation by Taliban fighters.
Hamid Karzai, the Afghan president, said he did not intend to re-create militias responsible for much of Afghanistan's misery.
"We want to strengthen districts so we get rid of the scourge of terrorism," Karzai told provincial delegates and elders on Sunday.
Thousands more NATO peacekeeping troops will be deployed by the end of July, and this summer is regarded as a critical period.
A decision to put irregular forces into the fray would run counter to a disarmament programme supposed to finish next year.
Abdul Manan Farahi, the interior ministry's counter-terrorism chief, told Reuters that the government would not arm the tribesmen, as they owned their own, but would register their guns and pay them for acting as a local defence force.
In eastern Kunar province, where US-led coalition forces launched Operation Mountain Lion earlier this year to clear out Taliban and al-Qaeda fighters, it has already happened.
"A tribal militia force was formed more than 10 months ago here," Zahidullah Zahid, a spokesman for Kunar's governor, said.
"These tribal people know the territory far better than the police and army who are sent from elsewhere."
Thousands more peacekeeping
troops are to be deployed
They own their own guns, mostly AK47s, most do not wear uniforms and they are paid around 4,000 afghanis ($80) a month by the interior ministry.
In southern Helmand province where British troops are stationed, former governor Sher Muhammad Akhundzada says he has enlisted several hundred tribesmen.
"I have raised 500 people and am working on their registration. The finance ministry pays them $200 a month," Akhundzada said.
Some members of the international community and factional commanders who have disarmed are angered by long-term dangers.
"This shows double standards," said Mohammad Faqir, who commanded 300 men in the northern city of Mazar-i-Sharif until they were disarmed last year.
"It is collecting arms from the north but giving arms to others in the south."
NATO spokesman in Kabul Mark Laity said he was unaware of plans to use irregular forces but said the disarmament of illegal armed groups, known as the DIAG process, was subject to local conditions.
"We can't have irregular armed forces, but this is very much a phased process," Laity said. "We are still committed to ensuring the DIAG policy is complete by the end of 2007.
"Implementation of that is obviously conditional on local situations to make it effective."
Gunmen robbed and shot dead four men working for an Indian road construction company in southern Afghanistan, a provincial government spokesman said on Sunday.
The men were Afghan nationals who had been living in Pakistan for about two decades as refugees and had returned to Afghanistan to work, officials in Pakistan said.
They were killed on Saturday in a district of Kandahar province known to be a stronghold of the Taliban movement that has been waging an insurgency since being removed from government in late 2001, spokesman Daud Ahmadi said.
It was too early to say whether the Taliban were behind the attack in Maiwand district, the spokesman said, telling AFP: "We don't know who the attackers were affiliated with."
The killers stole $80,000 and a four-wheel-drive vehicle the men had been travelling in, Ahmadi said.
The Taliban have admitted to several such killings in the past but have denied robbing their victims.
Nearly 30 foreign soldiers have been killed in hostile action in Afghanistan this year, more than half of them Americans.