It was the first incident of its kind in the south of the country and resembled attacks on police in Baghdad.

A senior British military source last night described the attack as “a complete change in tactics and not something we have seen down there before”.

The murders are a huge blow to British efforts to bring peace to southern Iraq, which hinge on training Iraqi security forces to enable them to take over responsibility for an area rife with illegal militia groups.

Such incidents could derail plans to reduce the number of British troops in southern Iraq.

The killings, and the deaths of another six police officers elsewhere in Iraq yesterday, could indicate the start of an all-out insurgent campaign against Iraq’s predominantly Shia security forces and an end to a post-Ramadan lull in attacks.

A senior Iraqi officer said that the gunmen had stopped the instructors in an area of Basra called al-Kibla. “They were killed and their bodies were taken back to Shuaiba and scattered around the town about four hours later. The two translators work with the instructors and the British Army and police,” he said.

Shuaiba is the town west of Basra that houses the large British base where more than 7,000 troops are based.

Major Charlie Burbridge, the British military spokesman for Iraq, said: “We are taking the incident extremely seriously indeed. The Iraqi police dealt with the situation from its first report and are continuing to investigate. We are poised to provide whatever the police request within our capabilities.”

Although the Basra area has been spared the sort of sectarian violence that is rife in central Iraq, the region is home to well-armed political, tribal and criminal factions.

Many officers are linked to militias and a clash between rival militias could have been behind the killings.

Last month Operation Sinbad was undertaken in a bid to rein in the militias, with commanders arrested, and to try to root out death squads in the police force. Major Burbridge said at the time that it was a “big push to take Basra as close as we can to transition to Iraqi control”.

Corruption within police ranks in southern Iraq has also been a serious problem for British forces. In September 2005, two undercover soldiers were caught by police and taken to Basra police station. The Army resorted to driving armoured vehicles through the building’s wall to free them.

Nouri al-Maliki, the Iraqi Prime Minister, wants to speed up the process of taking control of his own security forces, a wish that has been welcomed by the United States. Zalmay Khalilzad, the US Ambassador to Iraq, said that Mr al-Maliki wanted “more responsibility for the security of the country. We welcome his desire to have . . . more capable and credible forces and for him to have command and control of Iraqi forces.” he said.

Political tension deepened in Baghdad when Vice-President Tariq al-Hashemi, the country’s top ranking Sunni politician, threatened to resign if Mr al-Maliki did not act to eradicate two feared militia groups. He relies heavily on the backing of a pair of Shia political organisations and has resisted American pressure to eradicate their private armies — the Mahdi Army of the radical anti-American cleric Moqtada al-Sadr; and the Badr Brigade, the military wing of Iraq’s most powerful Shia political bloc, the Supreme Council for the Islamic Revolution in Iraq.

The gunmen, especially those of the Mahdi Army, are deeply involved in the sectarian killings that have brutalised Iraqis in Baghdad and central Iraq for months.

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