The NHS Covid app has sent a record number of contact-tracing alerts, informing people of their exposure to someone who has tested positive for the virus.
A total of 698,646 “pings” were sent to people in England and Wales between 16 December and 22 December.
That is nearly 8,000 more than the previous record of 690,711 sent during a seven-day period in July.
Data for the Christmas period is not yet available.
People “pinged” by the app with a “close contact” alert are advised to take daily Covid tests – or self-isolate if they are unvaccinated and over 18 – in order to reduce the likelihood of Covid transmission, but there is no legal requirement for them to do so.
During the summer, the app caused concern when large numbers of workers in various industries received pings, leaving them temporarily unable to do their jobs while they isolated.
A poll conducted at the time suggested a third of 18-34-year-olds had deleted the app, with another third planning to do so.
On Twitter, some users have recently complained of receiving an alert more than a week after exposure to someone with Covid, reducing the usefulness of the service.
The UK Health Security Agency has not responded to a BBC request for comment on this point.
But a spokesperson said: “The NHS Covid-19 app has prevented thousands of cases and is a vital tool to help protect against the spread of Covid-19 by alerting people when they may have been in contact with a confirmed case.”
“The app is an essential part of the pandemic response… we encourage everyone to continue using it as another tool to help keep us all safe.”