Many fully-vaccinated people could be asked to start self-isolating within the next 24 hours as officials examine positive Covid-19 swabs for the omicron variant.

Boris Johnson announced on Saturday that contacts of those who test positive for the new mutation will be asked to self-isolate for 10 days regardless of their vaccination status.

Public health officials are now carrying out “surge” testing in the areas of Essex and Nottingham where the first confirmed UK omicron cases were discovered on Saturday. Genome sequencing will be carried out on each positive case to identify further instances of the new mutation.

Under existing rules, people who had come into contact with those positive cases would not have been asked to self-isolate if they had received two or more doses of a Covid-19 vaccination. 

But, from later this week, stricter rules will be imposed on contacts of those who test positive for the omicron variant – meaning that hundreds or even thousands of people may be asked to self-isolate immediately as the sequencing work identifies the mutation among existing cases.

Speaking at a Downing Street press conference, Mr Johnson said: “We will require all contacts of those who test positive with a suspected case of omicron to self-isolate for 10 days, regardless of your vaccination status.”

Positive cases of the new variant were recorded in South Africa last week following genomic sequencing, which identifies a virus’s genetic make-up.

Deenan Pillay, a professor of virology at the University College London, told ITV News: “There has been an increase in South Africa in a number of provinces over the last few days.

“Only a small proportion of new Covid infections go through genetic sequencing, so the numbers could be higher.

“But it is premature to be scaremongering. We will learn quite soon whether there is any evidence of this spreading more generally.”