Britain as a whole has reported 11 cases of the new variant, which the World Health Organisation said on Monday was likely to spread internationally and posed a very high risk of infection surges.

Prime Minister Boris Johnson has responded to the emergence of Omicron by making mask-wearing compulsory in shops and on public transport in England.

He also asked the Joint Committee on Vaccination and Immunisation to review the booster program, which is open to over-40s, the vulnerable and health workers.

The independent advisory group recommended on Monday that boosters be offered to all adults under 40, that the minimum gap between second doses and boosters could be reduced to three months from six, and that 12- to 15-year-olds could get a second shot.

Health Secretary Sajid Javid said he would follow the advice and those who were most vulnerable would be prioritised for shots, with further details in the coming days.

Scotland reported six new Omicron cases on Monday, with two further cases confirmed in London, to bring the English total to five.

The four nations of the United Kingdom have restricted travel to southern Africa, where the variant was first detected last week, in a bid to slow its spread.

But Scottish First Minister Nicola Sturgeon said not all of the six cases had recent travel history or known links with others who had travelled to southern Africa.

"This suggests that there might already be some community transmission of this variant," she told a news conference.

Australian Associated Press