People should save lateral flow tests until just before meeting others, a scientist has advised.
The Omicron variant's infectiousness should push people to take a test before hooking up with others.
Public heath expert 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".
But 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 showed there had been 10,059 additional confirmed cases of the Omicron variant of Covid-19 reported across the UK.
This takes the total confirmed cases of the variant across the four nations to 24,968.
Omicron's rapid spread means tougher restrictions could soon be introduced after experts warned there are likely already hundreds of thousands of new infections every day.
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.
Earlier today The Mirror told about scientists from SAGE who warned toughers restrictions would be required as the variant spreads across the UK
Scientists said ‘Plan B’ measures alone would not be enough to prevent the variant ravaging the NHS - and recommended closing indoor pubs and cancelling crowded events.
Experts have estimated there are likely already hundreds of thousands of new infections every day.
And the number of Omicron cases recorded across the country officially hit almost 25,000 as of 6pm on Friday.
This figure was up by more than 10,000 cases from 24 hours earlier, the UK Health Security Agency (UKHSA) said.
The Mirror yesterday told how a "major incident" had been declared in London as Omicron raged across the capital.
It is the second time this year a major incident has been declared in London because of coronavirus.
The UK's eight worst Covid hotspots are now in London, where the variant has become the dominant strain, causing the overall cases to double in just a week.