The Centers for Disease Control and Prevention (CDC) and State Department officials on Monday issued an advisory to avoid travel to Britain due to a surge in coronavirus disease 2019 infections in the country.

The “Level 4” warning – the most serious issued by the U.S. agencies – came as the British government eliminated most of the pandemic restrictions in England even as new infections surged to levels not seen since January, driven by the Delta variant.

Both the CDC and the State Department urged U.S. residents to completely avoid travel to Britain, which joined Brazil, South Africa and the Netherlands on the Level 4 list.

“If you must travel to the United Kingdom, make sure you are fully vaccinated before travel,” the CDC said in its advisory. “Because of the current situation in the United Kingdom, even fully vaccinated travelers may be at risk for getting and spreading Covid-19 variants.”

Monday marked what the government of Prime Minister Boris Johnson called “freedom day,” when most Covid-19 restrictions in England were lifted, including social-distancing rules for public transport, pubs and bars.

But it came amid a surging caseload for Delta variants. In the past week, infections in Britain increased by more than 313,000, or 44 percent, including nearly 55,000 on Saturday, the highest daily level since January.

Fatalities also rose 39 percent, adding 283 in one week. Cases are multiplying despite nearly 70 percent of Britain’s population receiving at least one vaccine dose.

Johnson, who was hospitalized with coronavirus last year, is self-isolating after becoming exposed to his infected health secretary.

U.S. citizens are currently allowed to travel to Britain but face mandatory testing and quarantine requirements, while travelers returning to the United States from Britain must show a Covid-19 negative test or recent recovery from the disease before boarding a U.S.-bound flight.