"Obviously, it's an unusual find," said Maj. Mike Edwards, a British military spokesman in Kuwait City. "Who they belonged to or how they got there is under investigation."
There was no immediate indication of the nationalities of the dead, and it appeared they had died some time ago -- not in the current fighting.
Southern Iraq has been the site of bloody battles for over the past two decades, with heavy casualties in the 1980-1988 Iran-Iraq war; the 1991 Gulf War; and in the Shiite Muslim uprising that followed the Gulf War.
The estimated 200 coffin-sized boxes "contain bags, each labeled, and there's human remains inside the bags," said Capt. Jack Kemp of the Royal Horse Artillery. He told reporters at the scene that the remains were found by British forces who were searching the area within a half mile of one of their artillery pieces.
"I really wouldn't like to speculate on where the bones have come from," Kemp said. He said the remains were decomposed, and thus were not casualties of the current war.
Lt. Col. Ronnie McCourt, a spokesman for British forces in Doha, Qatar, confirmed that British troops found some bodies between Basra and Zubayr.
Associated Press Television News video showed the boxes stacked five high on one side of the warehouse, and other boxes were lined up on the other side. The video showed one skull that was missing front teeth and had a large hole in the nasal area. A British soldier held up folders containing lists written in Arabic.
British soldiers also discovered a catalogue of photographs of the dead. The relationship between the photographs and the remains was not immediately clear.
Faces in the photographs were burned, mutilated or scarred by grotesque wounds, according to Vanessa Allen, a reporter for Britain's Press Association news agency. Some apparently had been shot in the head, she added.
After permitting some journalists to look around the warehouse, British forces sealed the facility, Allen reported.
"We have placed it out of bounds to all personnel and we will treat it as a mass grave," Kemp said.
Zubayr is about 10 miles southwest of Basra, Iraq's second-largest city with a population of 1.5 million. British forces are seeking to root out resistance in Basra but have refrained from a direct assault.