A dead body has been found floating in the water tank of a US hotel, horrifying residents who were staying there.

British tourist Michael Baugh and his wife said water had only trickled for days as they brushed their teeth, showered and drank from the taps at the Cecil Hotel in downtown Los Angeles.

The body of a Canadian woman was later discovered at the bottom of one of four cisterns on the roof of the historic hotel near the homeless area known as Skid Row.

The tanks provide water for hotel taps and would have been used by guests for washing and drinking.

"The moment we found out, we felt a bit sick to the stomach, quite literally," Mr Baugh said.

Los Angeles County Department of Public Health officials are expected to release the results of tests on the water later.

When the body of 21-year-old Elisa Lam was discovered on Tuesday, officials issued a do-not-drink order while a lab analyses the water, said Terrance Powell, a director coordinating the department's response.

The disclosure contradicts a previous police statement that the water had been deemed safe.

The water was also used for cooking in the hotel. A coffee shop in the hotel will remain closed and has been instructed to sanitise its food equipment before re-opening.

The likelihood of contamination is considered minimal given the large amount of water the body was found in, but the department is being extra cautious.

Ms Lam's remains were found by a maintenance worker after guests complained about the low water pressure.

Police detectives are working to determine if her death was the result of foul play or an accident.

Her body was found on Tuesday morning at the bottom of one cistern that was about three-quarters full of water.

The opening at the top of the cistern is too small to accommodate firefighters and equipment, so they had to cut a hole in the storage tank to recover Ms Lam's body.

The cisterns are on a platform at least three metres above the roof.