I was in an office in Kabul this summer being lectured by a mid-ranking official about the successful work of the government. "Completely off the record, what do you really think of this government?" I asked him, not expecting a very interesting reply.

"So long as you promise not to reveal my identity, I can tell you that this government is made up of killers and crooks," answered the official with scarcely a pause. He gave some examples of government-inspired killings and corruption.

In this tradition of carefully calculated treachery, the shooting dead of five British soldiers by an Afghan policeman operating with them is hardly surprising. Afghan leaders have long been notorious for concealing their true loyalties and changing sides. But the potential political consequences are very serious. The US and British strategy to build up the Afghan security forces to as many as 400,000 may prove impossible because the state is too weak and too poor and commands the loyalty of too few Afghans.

The reputation of Afghans for always defeating their enemies is based in part on the speed with which they join the winner. The Taliban advances in the 1990s were notable less for military victories than local warlords defecting to them after receiving a large bribe. In the US war to overthrow the Taliban in 2001, the same process went into reverse as the CIA bought off the same warlords who then sent their men home without a fight.

Nor is this the first time that Western forces have been turned on by their Afghan colleagues. In Kunduz province north of Kabul earlier this summer, a policeman shot eight of his colleagues and turned his police post over to the Taliban. An American military trainer was shot and wounded by one of the men he was training when he drank water in front of them when they were fasting during Ramadan.

The shaky loyalty of the Afghan police and, to a lesser extent, the army to their own government undermines US and British plans to hold the line against the Taliban while a strong local security force is built up. US political leaders speak of a force of 240,000 soldiers and 160,000 police to be trained in the next few years. In reality, though, nobody knows the current size of the Afghan security forces.

The army is supposedly 90,000 strong, but this figure may be grossly over-stated. "My educated guess is that such an army simply does not exist," writes Ann Jones, an American specialist on Afghanistan. "I knew men who repeatedly went through ANA [Afghan National Army] training to get the promised Kalashnikov and the pay. Then they went home for a while and often returned some weeks later to enlist under a different name."

Even so, the reputation of the army among ordinary Afghans is much better than that of the police. Some of these are paid a pittance for a very dangerous job. They are often stationed in vulnerable outposts and checkpoints. Their training is frequently almost non-existent. Before the presidential election in August, policemen being trained by a US security firm who had been receiving eight weeks' training saw this reduced to three weeks, so they could be sent to guard polling stations in southern Afghanistan.

More senior policemen can make money through aiding drug smugglers. General Aminullah Amarkhail, the former head of security at Kabul airport, who was sacked for his success in arresting heroin smugglers, says that the profits are such that jobs are bought and sold for large sums. "You have to pay $10,000 [£6,000] in bribes to get a job as a district police chief," he says, "and up to $150,000 to get a job as chief of police anywhere on the border – because there you can make a lot of money."