Speaking at a Downing Street briefing, he also confirmed the reintroduction of face masks on public transport and shops, although they will not be required in hospitality venues.
Mr Johnson said the Government needed to take “targeted and proportionate measures” to curb the spread of the variant which scientists believe could be more transmissible and vaccine resistant. He stressed the new measures would be “temporary and precautionary” and would be reviewed in three weeks.
Ministers will also attempt to expand the booster vaccination campaign in the coming weeks, with health secretary Sajid Javid set to ask the Joint Committee on Vaccination and Immunisation (JCVI) to consider giving a third dose to those aged between 18 and 39. At present, only over-40s are eligible for a booster shot.
Senior scientists have said it is the worst strain they have seen so far during the pandemic and a string of countries have banned flights from southern Africa to prevent its spread.
Mr Johnson said: “We need to slow down the spread of this variant in our country and buy time for our scientists to find out what we’re dealing with.
“We don’t yet exactly know how effective our vaccines will be against Omicron but we have good reasons for believing they will provide at least some measure of protection.”
However, he reassured Britons that the Christmas period would be “considerably better” than last year, when swathes of country were plunged into lockdown amid the rapid spread of the Alpha variant.
Chief medical officer Professor Chris Whitty said there was a “reasonable chance” that there will be “some degree of vaccine escape” with the Omicron variant - but stressed more data would be needed for scientists to draw conclusions.
Elsewhere, he made a plea for Britons to “raise a glass” to scientists this Christmas in recognition of their contributions during the pandemic, adding that the country would be “in a very different place” without them.
Chief scientific adviser Sir Patrick Vallace said the UK may need to “face up” to the possibility of further action if the Omicron variant is very transmissible.
He said: “If it’s very transmissible and does cause big escape, then clearly that’s a major issue we have to face up to.
“But that isn’t what we know at the moment, we need to get that information.”
It comes after the first two cases of the variant were discovered in the UK on Saturday afternoon. Britain is now the second country in Europe, after Belgium, to report the presence of the strain.
Essex County Council said the cases, which are linked, are in Brentwood and Nottingham and those who tested positive are self-isolating and contact tracing is being carried out.
On Saturday evening, the Welsh government confirmed it would introduce the same measures on travel as England.