The warning -- the largest in the city's history -- impacts about 670,000 residents and will remain in effect until further notice.

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(CBS News) Local officials in Portland, Ore., issued a citywide boil notice after state health department detected a strain of E. coli in the water supply.

The warning -- the largest in the city's history -- impacts about 670,000 residents and will remain in effect until further notice.

The Portland Water Bureau said residents should boil all tap water used for drinking, food preparation, tooth brushing and ice for at least one minute.

Ice or any beverages prepared with unboiled tap water on or after Tuesday, May 20, should be discarded.

The three water samples that tested positive for bacteria were collected from reservoirs 1 and 5 at Mount Tabor.

Tri-County Health Officer Dr. Paul Lewis says the E. coli is unlikely to cause any health problems, except for maybe diarrhea. "Overall this is a very cautions approach," said Dr. Lewis during a press conference Friday afternoon. "We think the risk to the public is very low."

Lewis stressed the strain of E. coli is different from the microorganisms that have recently caused a number of food recalls, such as ground beef in 10 states. "But it's a marker of some kind of animal fecal contamination," he said.

The Water Bureau routinely tests 240 water samples each month. The agency says both reservoirs linked to positive samples will be emptied, flushed and refilled. City officials are currently investigating the cause of the contamination, and expressed hope that the notice could be lifted early tomorrow.

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