A public health expert has warned that lateral flow tests 'expire very quickly' and encouraged the government to update its guidance.
They also said the infectiousness of the Omicron variant should push people to wait to use lateral flow tests until just before meeting up with others.
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".
Current guidance from the government recommends taking a test "if you will be in a high-risk situation that day".
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However, Prof 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."
It comes amid reports a lockdown is being considered from December 27 to combat the growing surge in cases.
The Health Secretary said on Sunday that there are "no guarantees" more stringent restrictions would be introduced following a "sobering analysis" from scientific advisers warning about the threat from Omicron.
Sajid Javid said while there remains uncertainty about the new variant, it is time to be "more cautious" amid the rapid spread of the strain.
Mr Javid told Trevor Phillips On Sunday on Sky News that ministers are discussing the latest coronavirus data almost hourly with scientific advisers, and said the government will "do what is necessary" when asked about possible further measures.
He said: "We are watching the data and discussing it with our scientists and our best advisors almost on an hourly basis. And we will monitor that very carefully. We will keep the situation under review."
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, bringing the total confirmed cases of the variant across the four nations to 24,968.
Modelling from scientific advisers, published on Saturday, showed that if ministers stuck to the current Plan B measures, there could be a peak of 3,000 hospital admissions in England per day.
Mr Javid also said unvaccinated people are “taking up hospital beds” that could be used for someone else.
He said 10 percent of the population – more than five million people – still had not received their jabs, and around nine out of 10 of those needing the most care in hospital were unvaccinated.
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