The new measures came into place at 4am on Tuesday morning (30 November), with all travellers returning to the UK also required to take a PCR test and self-isolate until they receive a negative result.
All contacts of suspected Omicron cases must also self-isolate, regardless of their age or vaccination status.
Those aged 40 and over are already eligible for a booster jab.
The JCVI said booster doses should be given no sooner than three months after people have had their second dose of an original vaccine, which cuts three months off the current six-month wait.
Those aged 12 to 15 should be offered a second dose of the Pfizer/BioNTech Covid-19 vaccine, no sooner than 12 weeks after their first dose, the JCVI said.
The NHS said it will shortly set out how staff will expand the booster programme, including how booster jabs will be given in priority order so that the most vulnerable people are protected first.
‘Proportionate and responsible’
Prime Minister Boris Johnson said: “The measures taking effect today are proportionate and responsible, and will buy us time in the face of this new variant.
“Based on everything we know, our vaccines and boosters remain our best line of defence, so it is more important than ever that people come forward when eligible to get boosted.
“Not only will today’s steps help us slow down the variant’s spread, but they will help us protect each other and the gains we have all worked so hard for.”
The measures in place are described by Number 10 as “temporary and precautionary”, and will be reviewed in three weeks.