The US State Department and the US Centers for Disease Control and Prevention (CDC) both issued on Monday their highest warnings against travel to the United Kingdom because of a rising number of COVID-19 cases in that country.

Each raised the UK to "Level Four," telling Americans they should avoid travel there.

"If you must travel to the United Kingdom, make sure you are fully vaccinated before travel," the CDC said in an advisory, while the State Department said: "Do not travel to the United Kingdom due to COVID-19."

In May, the US government had lowered the UK to a "Level 3" advisory rating.

COVID-19 cases are rising by more than 50,000 a day in the UK and hundreds of thousands of Britons are being asked to self-isolate for 10 days.

The United States since March 2020 has barred nearly all non-US citizens who have recently been in the UK from the United States.

Britain allows American visits but requires a 10-day quarantine on arrival and two COVID-19 tests.

In June, the Biden administration said it was forming expert working groups with Britain, Canada, Mexico and the European Union to determine how best to restart travel safely after more than a year of restrictions.

US and airline officials do not expect the restrictions on UK travelers to be lifted until August at the earliest - and warn it could be pushed back further.

Airlines and others have pressed the administration to lift the restrictions that bar most non-US citizens who have been in Britain, the 26 Schengen nations in Europe without border controls, Ireland, China, India, South Africa, Iran and Brazil within the past 14 days from the United States.