Tullamore General Hospital has suspended the intake of patients for elective surgery, and emergency medical and surgical cases are being referred to Mullingar and Portlaoise hospitals, because of a continuing problem with the water supply.
Last week contamination was found in one of the water storage tanks. Work was carried out and appeared to have resolved the problem.
However, when water from the tank was tested again this week it was found to be contaminated again. A health board spokeswoman said last night that work had recommenced.
She said there was no contamination of the hospital's main water supply. Drinking water was available throughout the hospital, she said, and no cases of gastroenteritis, which could have resulted from consuming water from the tank, had occurred.
But, she said, because the water from the tank was used for washing in some of the theatres, it was decided to refer surgical cases to other hospitals in the area.
"The board's engineering staff are continuing to work on the tank to ensure that there is no contamination and we are ensuring that the water from the tank is safe before it is used again," she said.