Twenty one people have been confirmed as testing positive for coronavirus on board Grand Princess off the California coast.
They include two passengers and 19 members of the ship’s crew.
The results come from the first phase of health screenings of passengers and crew.
Princess Cruises was notified by US vice president Mike Pence and simultaneously by the US Centers for Disease Control (CDC) while he was conducting a press conference.
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More than 140 Britons are reported to be among the 2,422 passengers and 1,111 crew on the ship.
Grand Princess has been held off the coast of California after a passenger from a previous cruise died from the disease.
The US government is working with California officials on a plan to bring the ship to a non-commercial port this weekend where all passengers will be tested, Pence said.
He added that it was likely the crew members caught the virus on two different voyages of the ship and he expects all members of staff on the vessel to be quarantined.
The ship had been due to dock in San Francisco on Wednesday but has been held at sea.
A Princess Cruises spokesperson said: “The ship’s doctor is in the process of informing the guests and crew of their individual results.
“All guests and affected crew will remain isolated in their rooms.
“We are awaiting official specific plans for future positioning of the ship from relevant authorities.
“Princess Cruises will continue to closely follow the guidance of the CDC and other federal and state government authorities.
“Guests will continue to be provided complimentary internet and telephone to stay in contact with their families and loved ones, and the ship’s company is working to keep all guests comfortable.
“The ship has quickly transitioned food and beverage service to provide room service for all staterooms.
“We are working with CDC to determine what additional precautionary measures, if any, will be required.
“Princess Cruises medical team is collecting information from guests regarding any medication prescription refills needs.”
The line’s Diamond Princess was quarantined in Japan in early February for 27 days. Six of its passengers – including the first Briton – died after contracting the virus.