Abu Iman barely flinched when the Iraqi Government ordered his unit of special police to move against al-Mahdi Army fighters in Basra.

His response, while swift, was not what British and US military trainers who have spent the past five years schooling the Iraqi security forces would have hoped for. He and 15 of his comrades took off their uniforms, kept their government-issued rifles and went over to the other side without a second thought.

Such turncoats are the thread that could unravel the British Army’s policy in southern Iraq. The military hoped that local forces would be able to combat extremists and allow the Army to withdraw gradually from the battle-scarred and untamed oil city that has fallen under the sway of Islamic fundamentalists, oil smugglers and petty tribal warlords. But if the British taught the police to shoot straight, they failed to instil a sense of unwavering loyalty to the State.

“We know the outcome of the fighting in advance because we already defeated the British in the streets of Basra and forced them to withdraw to their base,” Abu Iman told The Times.

“If we go back a bit, everyone remembers the fight with the US in Najaf and the damage and defeat we inflicted on them. Do you think the Iraqi Army is better than those armies? We are right and the Government is wrong. [Nouri al] Maliki [the Iraqi Prime Minister] is driving his Government into the ground.”

The reason for his apparent switch of sides was simple: the 36-year-old was already a member of the al-Mahdi Army which, like other militias, has massively infiltrated the British-trained police force in the southern oil city. He claimed that hundreds of others from the 16,000-strong force have also defected to the rebels’ ranks.Abu Iman joined the new Iraqi police force after the invasion, joining the Mugawil, a special police unit infamous for brutality, kidnapping and sectarian murders.

“We already heard two weeks ago that we were going to attack the Mahdi Army, so we were ready,” he said. “I decided to take off my uniform and join my brothers and friends in the Mahdi Army. All these years, we were like a scream in the face of the dictator and the occupation.” He said: “I joined the police because I believed we have to protect Basra and save it with our own hands. You can see we were the first fighters to take on Sadd-am and his regime, the best example being the Shabaniya uprising.”

Abu Iman said that the fighting raging in Basra yesterday was intense because the al-Mahdi Army was operating on its own turf. He was confident that the Shia militia would prevail because its cause was just.

“The Iraqi Army is already defeated from within. They come to Basra with fear in their hearts, knowing they have to fight their brothers, the sons of Iraq, because of an order from Bush and his friends in the Iraq Government. For this reason, all of the battles are going in the Mahdi Army’s favour.”

Major-General Abdelaziz Moham-med Jassim, the director of operations at the Ministry of Defence, played down reports of defections in the Basra police force. “The problem of one policeman doesn’t make up for the whole of the force,” he said.

In recent months Major-General Abdul Jalil Khalaf, Basra’s police chief, has tried to shake up the force and drive out militia infiltrators, who have wrought havoc in the past, often turning police stations into torture cells in which factions settled vendettas and power struggles with murder and abuse. But he only narrowly escaped an assassination attempt yesterday when a suicide car bomb attack in Basra killed three of his policemen. A local tribal leader said the police directorate building was later gutted by fire.

Times Recommends

• Times Charity Appeal: SolarAid
• Is it Black Monday all over again for Dubai?
• Behind the scenes at the biggest deal