Olivia Rudgard, in San Francisco, reports on the latest from the Grand Princess cruise ship.

Staff from the US Centre for Disease Control arrived by helicopter on to a cruise ship off the coast of California carrying 140 British citizens, among the 3,400 passengers and crew trapped amid concerns that coronavirus may be spreading among them.

Videos taken by passengers show helicopters hovering above the Grand Princess at 11.30am on Thursday, and staff clad in masks and protective gear standing on deck.

Announcements made on the ship on Thursday morning ask passengers to practice "social distancing", by staying six feet away from each other, under recommendations from the CDC.

Previously allowed to move around the ship, they were asked to stay in their cabins after lunchtime on Thursday and told they would be receiving room service for all future meals.

Passengers were also given a hotline number to call if they began to feel unwell.

Cruise company Princess said samples were collected from "fewer than 100" guests and crew.

Samples are set to be sent to laboratories in San Francisco for testing and results are expected early Friday morning.

Social media posts suggest British passengers on board include crew, guest services staff and entertainers.

Family members of passengers expressed concerns about a repeat of the situation which emerged on the Diamond Princess, a cruise run by the same company, where the virus spread quickly among guests confined to their cabins off the coast of Japan.