The attack Tuesday afternoon came a month after an Afghan policeman on patrol with U.S. soldiers fired on the Americans, killing two. Training and operating jointly with Afghan police and soldiers is key to NATO's strategy of dealing with the spreading Taliban-led insurgency and, ultimately, allowing international forces to leave Afghanistan.
Attacks such as these will heighten concern about the effectiveness of the Afghan forces.
Lt. Col. David Wakefield, spokesman for the British forces, told Sky News that the soldiers had been mentoring Afghan national police and had been working and living in the police checkpoint in Helmand's Nad-e-Ali district.
"It is our initial understanding that an individual Afghan policeman possibly acting in conjunction with one other started firing inside the checkpoint before fleeing from the scene," he said.
A Helmand police official said authorities searched through the night and on Wednesday for the attacker. He said the man had been working as a police officer in the area for three years, and had passed through a police academy in Kandahar. The official, who spoke on condition his name not be used because of the sensitivity of the situation, said it was unclear what his motives were.
The attacker was on the roof of a police checkpoint and opened fire on the British soldiers, who returned fire. Six other soldiers were wounded, as were two Afghan policemen, NATO forces headquarters in Kabul said in a statement.
NATO said the attack was being investigated by NATO forces and Afghan authorities.
The British fatalities were among the largest in a single incident in Afghanistan. They brought the total number of British forces who have died in Afghanistan to 229. Britain has 9,000 troops in the country, the second largest force after the United States. Last month, British Prime Minister Gordon Brown announced plans to increase troop numbers by 500.
Former Foreign Minister Abdullah Abdullah, who was the main challenger to President Hamid Karzai in Afghanistan's recent fraud-marred election, said the continuing violence showed the Karzai administration had failed to bring peace to the country despite assistance from international forces.
"As far as the presence of international forces in Afghanistan is concerned, eight years of golden opportunity we have missed. You were here. Your soldiers were here, and they have made sacrifices for bringing peace and stability to Afghanistan," Abdullah said during a news conference in Kabul.
"But eight years down the road we still need more troops. In the absence of a credible and reliable and legitimate partner, more soldiers, more resources" are needed, he said.
Presidential spokesman Humayun Hamidzada said it was an isolated attack.
"These are incidents that can happen anywhere. The crazy man who has done this has also attacked the Afghan police," he told the AP. "You can't use this isolated incident to say that there is a problem with the police force of Afghanistan. In the U.S., people shoot up people in a shopping mall. There are crazy people everywhere."
Karzai issued a statement condemning the killings and offering condolences to the people of Britain and the relatives of the soldiers.
The commander of international forces in Afghanistan, Gen. Stanley McChrystal, said he discussed the shooting with Interior Minister Mohammad Hanif Atmar, who "gave me his assurance that this incident will be fully and transparently investigated."
"We will not let this event deter our resolve to building a partnership with the Afghan National Security Forces to provide for Afghanistan's future," he said in a joint statement issued by NATO forces and the ministry.
Atmar said the attack "appears to be an isolated incident."
Last year, Afghan policemen twice attacked American soldiers in the space of about month. In October 2008, a policeman threw a grenade and opened fire on a U.S. foot patrol, killing one soldier, while in September, an officer opened fire at a Paktia police station, killing a soldier and wounding three before he was fatally shot.
Peter Galbraith, the former top American official at the U.N. mission in Afghanistan who had called attention to fraud charges in the country's presidential election, told British radio that police training and recruiting had been "rushed" in Afghanistan.
"It is a terrible tragedy but it is, I won't quite say inevitable, but it is not surprising," he told BBC Radio 4.
"The process of police training and recruiting has been very rushed. Normally the police get an eight-week training course. That is actually very short and there isn't a lot of vetting of police before they are hired."
Such attacks have also occurred in Iraq, where U.S. and coalition forces are engaged in a similar process of mentoring and training the Iraqi army and police.
In February, two Iraqi policeman opened fire at a police outpost in northern Iraq, killing an American soldier and an interpreter and wounding three U.S. soldiers - the fourth attack since late 2007 with suspected links to Iraqi security units.
In London, Brown extended his condolences to the soldiers' families.
"They fought to make Afghanistan more secure, but above all to make Britain safer from the terrorism and extremism which continues to threaten us from the border areas of Afghanistan and Pakistan," he said.
Associated Press writers Noor Khan in Kandahar, Heidi Vogt in Kabul and Jennifer Quinn in London contributed to this report.