The Hospital is blaming a faulty seal on water storage tanks kept on the hospital roof.

The contamination of the tap water was discovered by routine checks and, as a health precaution, management at the hospital took the decision to order all 150 women patients and 400 staff members to drink only bottled water.

The Chief Executive Peter Herring insisted that there was very little risk of illness from the contaminated water.

He said no patients had been transferred and hospital business was continuing as usual. "It seems rainwater washed dirt into the tank and caused low-level contamination," he said.

"The system will be flushed with chlorinated water tonight and that means opening every tap in the hospital. That should guarantee there is no contamination left, but we have to wait for 24 hours afterwards."

Environmental health experts will carry out further tests on the water supply and local GPs in the Toxteth area of Liverpool have been informed about the situation. Mr Herring said he expected the bottled-water precaution to last until Saturday.