The UK on Sunday raised its COVID-19 alert level to level four, its second highest level, following a recommendation from the UK Health Security Agency (UKHSA) and amid a surge in cases of the Omicron variant.

Earlier on Sunday, the UK reported 1,239 additional cases of the Omicron variant, the highest daily rise and almost twice the previous highest number (633) reported on Saturday.

Level 4 means the COVID-19 epidemic "is in general circulation" and that "transmission is high or rising exponentially".

Therefore, health authorities believe there are high rates of transmission and healthcare services are coming under pressure.

The previous level, level 3, suggests the epidemic is "in general circulation", but does not include the statement "transmission is high or rising exponentially".

"Transmission of COVID-19 is already high in the community, mainly still driven by Delta, but the emergence of Omicron adds additional and rapidly increasing risk to the public and healthcare services," the government said in a statement.

"Early evidence shows that Omicron is spreading much faster than Delta and that vaccine protection against symptomatic disease from Omicron is reduced."

"The NHS is currently under pressure mainly driven by non-COVID pressures. With a variant spreading with increased transmissibility and reduced vaccine effectiveness, we are likely to see this pressure rise soon."

Meanwhile, UK prime minister Boris Johnson is expected to address the nation later on Sunday on booster vaccines.

(With input from agencies)