The Telegraph understands that the Government is considering allowing a Christmas window in which families can meet, with lockdowns on either side of the festive period.

Boris Johnson has so far refused to impose any new nationwide restrictions such as a “circuit-breaker” despite new national lockdowns in France, Italy and Germany, where infection rates are lower.

However, the technical changes to the NHS app – now downloaded by more than 19 million people, approximately 40 per cent of those with eligible phones – could send significantly enhanced numbers into de facto lockdown with no ministerial announcement.

Designers have also taken advantage of an upgrade to the Google/Apple technology that underpins the app to improve its ability to determine proximity between phones.

On top of improvements to the app’s ability to judge proximity, officials have lowered the risk threshold which determines whether an alert is sent from a score of 900 to one of 120.

A Department of Health spokesman said: “This is expected to increase the number of people asked to self-isolate by the app having been in close contact with someone who has tested positive.

“In the context of rising infection rates across the country, this change is necessary to break the chain of transmission, helping curb the spread of the virus and therefore ensuring fewer people are infected in the long term.”

Source Article