People across the UK will be allowed to meet with their families for the first time in months when Covid restrictions are eased at Christmas.
The rules mean a number of households can mix and gather indoors for five days from December 23 to December 27.
But a top scientist has warned that the only way to make the move safe is for people to isolate for 10 days before they head out to visit friends and family.
Former Government chief scientific adviser Sir David King has issued the because of alarming infection rates among children, reports The Mirror.
The relaxing of the rules applies in every part of the UK - regardless of restrictions which would normally stop indoor gatherings and household mixing.
Sir David said: “Many children will unwittingly have the disease. People will need to be exceptionally careful unless they isolate for between 10-12 days beforehand. There will be a price to pay for Christmas.”
University College London’s Prof Susan Michie of Independent Sage scientists said: “There is a big risk if younger people have not isolated before contact with older relatives.
“If parents know there are going to be vulnerable people around they should want children to self-isolate first.”
One in five children – 900,000 – are not in class either because they have the disease or are isolating with symptoms – a 50-fold increase on September.
Prof Stephen Reicher said Mr Johnson should have kept restrictions tight at Christmas. He said: “The spirit of Christmas should mean the love and strength to keep our distance.”
Prof Gabriel Scally said: “There’s no doubt Christmas will increase cases and result in deaths.”