(Updated: 12:30 p.m. BST, May 10th): Almost a third of passengers have now reported illness onboard Fred. Olsen’s Balmoral over the course of the ship's current voyage.

The Center for Disease Control and Prevention states that a total of 277 of 915 (30.27 percent) passengers have reported illness during the voyage so far, while nine of 520 crew members have reported illness.

However, Fred. Olsen Cruise Line has confirmed that there are currently 15 passengers who have been required to remain in their cabins because of a gastroenteritis type illness onboard.

A statement from the line confirmed that the ship will continue with its current itinerary as planned, before arriving back in Southampton on May 20, adding: "Balmoral has undergone inspections by the US Centers for Disease Control and Prevention ‎(CDC) and US Coastguard and received a US Public Health score of 91 percent. At no point has Balmoral been quarantined in any port on this cruise, and is continuing as planned."

(Updated: 1:00 p.m. EDT, May 4th): A further 20 passengers have reported illness overnight. Fred. Olsen emphasized that the illness had not at this stage been confirmed as norovirus, "but we wish to take every precaution," and cancel the planned visit. The statement added: "Prior to this there were zero cases reported for the previous 48-hour period, and we had no guests in isolation."

(9:40 a.m. EDT) -- An outbreak of norovirus has been confirmed onboard Fred. Olsen's Balmoral cruise ship, affecting passengers on the current 34-night Old England to New England voyage, which left Southampton on April 16.

An investigation by the U.S. Centers for Disease Control's Vessel Sanitation Program reveals that 163 of 921 passengers onboard the ship -- over 17 percent of passengers -- reported falling ill, with six of 518 crew members also falling ill.

Two environmental health officers from the Vessel Sanitation Program boarded the ship in Baltimore on April 30 and May 1 to carry out health assessments. The ship was then cleared by the CDC to continue to New York City, where it is currently docked.

Cruise Critic member nb125 who has been travelling onboard Balmoral wrote: "The Captain announced the outbreak and instigated the usual measure six days in to the cruise which seems to indicate it was not brought on board by anyone at the start of the cruise."

The same member later added: "It was not [confined to] one person, there were at least sixteen cabins effected and the captain actually called it norovirus whereas last year they insisted that it was a "gastric type illness". I am assuming from this that they had performed tests and confirmed it as norovirus."

A press conference was held when the ship docked in Norfolk on April 29, according to the Virginian-Pilot. At that time, the ship's captain told reporters that ill passengers were asked to stay in their cabin and that the ship did not dock in Bermuda because of the illness, the newspaper reported.

In a current statement from Fred. Olsen Cruise Lines, the line said that there are currently "just six guests in isolation, out of a total of 1,434 guests and crew onboard, and the incidences of gastric illness have reduced substantially.

"Fred. Olsen has been undertaking extensive sanitisation measures and cleaning of the ship, following the company's strict illness containment and prevention plan. It is believed that the highly-contagious gastric illness was brought onto the ship, and is spread by person-to-person or surface-to-surface contact."

With symptom including vomiting and diarrhea, norovirus outbreaks occur when many people are in a small area, including nursing homes, restaurants, hotels, dormitories and cruise ships. For more information, read Norovirus: What You Need to Know.

--By Kerry Spencer, Editor, U.K.