Residents in King County Water District 54 in Southwest King County were told to boil their tap water Sunday after routine tests discovered potentially harmful E. coli bacteria, the state Department of Health said in an advisory to 5,000 customers.

The water district extends south from South 212th Street to the Kent-Des Moines Road and east from Puget Sound to 14th Avenue South and includes parts of the cities of Des Moines and Normandy Park.

E. coli bacteria can cause gastrointestinal illness. Customers were urged to briskly boil water used for drinking, brushing teeth, preparing food, making ice and washing dishes for one minute, then letting it cool before using, according to the Department of Health.

Restaurants, coffee shops, delis and school kitchens in the affected area should stop operating until the advisory is lifted. Stores that sell only prepackaged foods may continue operations, the department said.

The water-district staff will disinfect the system with chlorine and flush it out of the water lines, then sample the water again. On Monday, a Department of Health engineer will inspect the system with water-district staff to look for the source of contamination, according to the department advisory.

“The safety of our customers is our top priority,” said King County Water District 54 Superintendent Eric Clarke. “We’re working closely with the state to resolve this problem as quickly as possible.”

The boil-water advisory will remain in effect until the Department of Health and the water district are satisfied the water meets safe drinking-water standards.

Customers who have questions about their water quality can call the water district, 206-878-7210.