SAGE scientists urged Boris Johnson to keep free Covid testing as he plans to scrap isolation rules in a bid to live with the virus.
Ministers are expected to phase out mass testing in the coming months.
But the Sage panel, headed by Patrick Vallance and Chris Whitty, said it would lead to a spike in infections and new variants spreading by stealth.
Sage warned: “This suggests there is significant potential for transmission to increase if behaviours revert rapidly to pre-pandemic norms and mitigations are removed.
“Removing access to free testing would make it harder for people to take precautionary actions.
“It may also increase anxiety among those who have found testing reassuring after possible exposure, particularly those who are or live with someone who is clinically vulnerable.
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“Some people may also take the removal of free testing as a signal that they should continue to attend workplaces or social gatherings while showing Covid-19 symptoms.”
Sage said the public must be told that isolating while sick is still firm medical advice even when it is no longer the law.
The scientists said the faster spreading BA.2 variant, an Omicron offshoot that now accounts for one in five cases in the UK, could make cases surge more if testing is scrapped.
They added: “The emergence of new variants and a resultant wave of infections can occur very quickly, potentially within just several weeks.
“The ability to rapidly detect and characterise new variants will be very important.”