People are being urged to take a rapid Covid-19 test before mixing with others in “crowded indoor spaces”.
Previously the public was advised to use lateral flow tests twice-weekly.
But Government and NHS guidance on when to take a test has changed and it now urges people to take a test before mixing with people in crowded indoor places or before visiting a person who is at higher risk of getting seriously ill from Covid-19.
The advice comes ahead of the busy festive period where people will be spending more time seeing loved ones, shopping or going to Christmas parties.
People who develop symptoms of Covid-19 – including a new and persistent cough, a fever or a loss or change of taste or smell – are still encouraged to self-isolate and get a lab test, also known as a PCR test.
The advice on the Government website says that around one in three people who have Covid-19 do not have any symptoms, which means they could be spreading the virus without knowing.
It urges people to use the free rapid lateral flow tests, which can be collected from pharmacies and are available online.
The advice adds: “You are at higher risk of catching or passing on Covid-19 in crowded and enclosed spaces, where there are more people who might be infectious and where there is limited fresh air.
“You may wish to take a rapid lateral flow test if it is expected that there will be a period of high risk that day.
“This includes spending time in crowded and enclosed spaces, or before visiting people who are at higher risk of severe illness if they get Covid-19.”
It adds that certain places such as health and social care settings, schools and prisons are likely to have their own specific testing rules and guidelines.
The guidance also urges people to take other precautions including letting fresh air in for indoor meetings and to wear face coverings in “crowded and enclosed areas where you come into contact with people you do not usually meet”.
A spokesperson for the UK Health Security Agency said: “About one in three people with Covid-19 do not have any symptoms but can still infect others.
“Rapid testing plays a critical role in uncovering hidden cases of Covid-19 and preventing the spread of the virus.
“Taking a free rapid lateral flow test before a period of high risk will give you peace of mind that you are unlikely to be infectious and able to spread Covid-19.
“It remains vital to get vaccinated and continue to take other preventative measures such as regular hand washing, wearing a face mask and keeping spaces ventilated when meeting people indoors.”
It comes as Mark Woolhouse, professor of infectious disease epidemiology at the University of Edinburgh, told BBC Radio 4’s World at One programme that the UK was “further ahead with our Delta wave than Europe” and was “not expecting to see the explosive increasing cases that they have seen in parts of Europe recently”.
He said there was a need to “stay very vigilant and respond to what we see in front of us”.
Asked if the message was that people should “buy the turkey and invite the family over”, Prof Woolhouse said he was “optimistic but not complacent”.