Last September, British troops battled Shiite gunmen in Basra after two British undercover soldiers were seized by police, whose ranks have been infiltrated by Shiite militiamen. British forces staged a raid that freed the men.

Tensions boiled over again in February when the London newspaper News of the World published video images that appeared to show British soldiers beating Iraqi civilians during a riot in Amarah in 2004.

Shiite anger also has been stoked by a perceived shift in U.S. policy since the arrival of U.S. Ambassador Zalmay Khalilzad, a Sunni Muslim who has criticized the Shiite-led Interior Ministry for human rights abuses and made overtures to Sunni insurgents in hopes of getting them to lay down their arms.

Two British soldiers were killed and two others wounded in a roadside bombing in Basra on Sunday, bringing to nine the number of British personnel who have died in the city this month and pushing total British deaths in the past three years to 113. American deaths, meanwhile, are approaching 2,500.

Sunni leaders also ordered the closure of all Sunni mosques in the city and urged preachers not to hold Friday prayers last week to protest the killing of a local Sunni cleric.

Wafiq al-Hamadani, the imam of Kawaz mosque in Basra, was slain Friday when gunmen in a speeding car opened fire on him as he was walking to his mosque.