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."
Spike in cases
The NHS app also allows people to report the results of a Covid test.
Reports of positive test results also reached a new high – 243,406 across England and Wales during the same seven-day period ending on 22 December.
Not everyone who receives a positive result uses the app or will necessarily upload their result.
Official data shows that record numbers of positive cases have been recorded across the UK during recent days, with 189,213 added on 30 December alone.
Separate contact-tracing apps are used in Scotland and Northern Ireland. The Scotland data also suggests a record number of contact notifications were sent via the Protect Scotland app during the period.
The BBC has also contacted the Department of Health in Northern Ireland for comment.