A stomach bug causing vomiting and diarrhea has spread to more than a quarter of the 919 passengers aboard a British cruise ship, the U.S. Centers for Disease Control (CDC) said, as the ship docked in Maine over the weekend.

It also said eight of the 520 crew on the Balmoral, operated by Fred. Olsen Cruise Lines, had also fallen ill with the bug, identified as a norovirus.

The Balmoral left Southampton, England on April 16 for a 34-day cruise, making stops in Portugal and Bermuda before putting in at Norfolk, Virginia, where it first arrived in the United States late last month.

CDC officials said at that time that 153 passengers and six crew had been infected by norovirus. Health officials and an epidemiologist boarded the ship at its next stop in Baltimore, Maryland to assess the outbreak and the response.

The CDC said specimens collected and onboard tested positive for norovirus, and would be sent to CDC for additional testing.

Fred. Olsen said in an April 29 statement that a "gastro-enteritis type illness" had affected a number of guests, with seven cases in isolation at that point.

It said two U.S. nationals were on board, with the majority of passengers from the United Kingdom.

When the Balmoral docked at Portland, Maine, over the weekend, media reported witnesses seeing surfaces being constantly wiped down.

The ship was due to stop at St. John, New Brunswick, Canada, on Monday.

CDC said the cruise line had taken actions in response to the outbreak, including increasing cleaning and disinfection procedures, collecting stool specimens, daily reporting of illness and dispatching public health and sanitation managers to oversee and assist with implementation of sanitation and outbreak response.

Balmoral has capacity for 1,350 passengers, and is the largest and newest ship in the cruise line's fleet. - Reuters