People should take lateral flow tests just before meeting up with others because Omicron can become infectious "within hours", a public health expert has warned.
Irene Petersen, a professor of epidemiology at University College London, said official advice should be updated as those infected with Omicron "may switch from being non-infectious to infectious within hours".
Government guidance currently recommends taking a test "if you will be in a high-risk situation that day".
Professor Petersen told The Sunday Telegraph: "That's not good enough. We're seeing so many examples now where people have taken a test a day before and then when they take one the day after they are positive.
"Omicron is very, very fast, so the test result expires very quickly. It is hours that we are talking about now."
The latest figures from the UK Health Security Agency, formerly Public Health England, showed there had been 10,059 additional confirmed cases of the Omicron variant of COVID-19 reported across the UK, bringing the total confirmed cases of the variant across the four nations to 24,968.
Watch: COVID-19: No PCR slots available in parts of England - with lateral flows still unavailable online
Ministers reportedly drawing up circuit-breaker plans
The variant's rapid spread means tougher restrictions could soon be introduced to prevent hospitals becoming overwhelmed.
Stricter measures could be imposed after Boxing Day, according to a report in The Sun newspaper, which said the contingency plan had not yet been presented to ministers.
Sky News understands experts who produce advice for ministers believe there are now "hundreds of thousands" of infections every day, many more than the number of official cases recorded on the daily dashboard.
Even if transmission of the variant stopped now, a significant number of hospital admissions are already baked in.
It comes as ministers are reportedly drawing up plans for a so-called circuit breaker.
Indoor mixing could be banned
Both The Times and Financial Times have detailed the potential new restrictions, which could come into force for two weeks after Christmas.
It's suggested that a return to the measures that we were living with in April - under step two of the prime minister's roadmap out of lockdown - would be needed to stop hospitals being overwhelmed.
Pubs and restaurants could only serve at tables outside, and indoor mixing with other households was banned.
The source said the circuit breaker would need to last for long enough for everyone to get a booster, building an extra wall of defence.
Watch: COVID-19 - Scientists 'surprised' by properties of Omicron variant, says NERVTAG expert