The updated recommendation came from the US Centre for Disease Control and Prevention (CDC) on Friday (20 August).

Those with an increased risk of severe illness, regardless of vaccination status, and those who have not been fully vaccinated have been advised to avoid travel on cruise ships, including river cruises, worldwide.

The CDC warned it may adjust requirements and recommendations on its phased approached to returning US cruise ships back into operation "based on public health considerations and other factors".

Cruises are currently ranked level three in the CDC’s list of Covid risk levels.

The organisation said: "The virus that causes Covid-19 spreads easily between people in close quarters aboard ships, and the chance of getting Covid-19 on cruise ships is high.

"People who decide to go on a cruise should get tested one to three days before their trip and three to five days after their trip, regardless of vaccination status.

"Along with testing, passengers who are not fully vaccinated should self-quarantine for seven days after cruise travel, even if they test negative. If they do not get tested, they should self-quarantine for 10 days after cruise travel."