Thirteen cases of the variant, which may be more infectious, have been found in England with a further nine in Scotland, Boris Johnson told a Downing Street press conference on Tuesday afternoon.
Health chiefs fear existing vaccines could be less effective against the new mutation.
Health Secretary Sajid Javid said it was not known whether the most recent Omicron cases identified in the UK were linked to travel in South Africa.
He told the press conference: “The cases that we identified at the start of the weekend, we were able to confirm quickly that they were linked to travel to South Africa. They were also part of one sort of family cluster.
“The recent cases that we’ve confirmed today, we’re not able to say whether or not, at this point … we don’t know, we’re doing that work right now at speed to determine whether they all have a link to South Africa or not.
“Is there likely to be community transmission? I think we have to be realistic, there is likely to be, as we’re seeing in other European countries.”
UK Health Security Agency chief Dr Jenny Harries updated Cabinet on Tuesday morning on the latest coronavirus developments, along with England’s chief medical officer (CMO) Professor Chris Whitty and chief scientific adviser (CSA) Sir Patrick Vallance.
The Prime Minister’s official spokesman said: “While further time is needed to study the characteristics of this variant and with work ongoing at Porton Down to assess it, both the CMO and CSA emphasised that getting a booster vaccine was the best way to gain protection against new and existing variants.”
The Prime Minister told Cabinet that the Government had taken “decisive and proportionate action against the new variant to slow the seeding and potential spread of Omicron and that our most important defence remains vaccinations and boosting our booster programme”, the spokesman said.
Booster jabs are to be offered to all over-18s in the UK, as part of efforts to bolster the vaccination programme, with children aged 12 to 15 invited for a second jab.