Two major sporting events held in March "caused increased suffering and death", the scientist leading the UK's largest Covid-19 tracking project has said.
Data gathered from millions of volunteers found coronavirus "hotspots" shortly after the Cheltenham Festival and Liverpool's Champions League match against Atletico Madrid.
Professor Tim Spector said rates of cases locally "increased several-fold".
The government said many factors could influence cases in a particular area.
Less than three months ago sport across the UK was continuing as normal, despite the impending threat of coronavirus - which had already prompted some European countries to stage such events without spectators, or completely call them off.