Data shows 618,903 alerts were sent - a 17% rise from the previous week.
There are complaints from businesses that the alerts are causing serious staff shortages and affecting services.
However some fully-vaccinated key workers will be exempt from self-isolating if they are pinged by the app as a close contact of a positive case.
Business secretary Kwasi Kwarteng told the BBC a “very narrow” list of jobs would be released today.
