On Sunday evening, Boris Johnson urged the British public to "let's get boosted now".
In a pre-recorded statement, the prime minister spoke of "a tidal wave of Omicron" and "an emergency". The target of giving booster vaccine jabs to all adults, he said, will be brought forward by a month from the end of January.
Boris Johnson explained that two doses of vaccine were simply not enough to give the level of protection needed. "But the good news is that our scientists are confident that with a third dose, a booster dose, we can all bring our level of protection back up."
Earlier on Sunday, the UK's Covid alert level was raised to four due to the spread of the Omicron variant. Level four means a high or rising level of transmission. The last time the UK was at level four was in May.
The prime minister did not announce any further Covid-related restrictions.