Liverpool's Champions League game with Atletico Madrid and racing's showpiece Cheltenham Festival "caused increased suffering and death" by going ahead in March, a leading scientist told the BBC on Tuesday.
Professor Tim Spector, who leads the United Kingdom's largest Covid-19 tracking project, said rates of cases locally "increased several-fold" following the events.
Cheltenham attracted over 250,000 spectators to southwest England on March 10-13 and Liverpool played the Spanish side at Anfield on March 11 in front of a 52,000 crowd.
Figures seen by the BBC show in the last week of March, Liverpool and Cheltenham were among the areas with the highest number of suspected cases.
The figure from the Covid-19 Symptom Study show an estimated five to six percent of the population, aged 20 to 69, having symptoms in those two regions.
The British government had advised that people should feel free to attend such events despite governments of other countries cancelling events at that time.
Prime Minister Boris Johnson attended England's Six Nations home game with Wales along with his then pregnant girlfriend Carrie Symonds on March 7.
Britain's Culture Secretary Oliver Dowden said at the time there was "no reason for people not to attend such events or to cancel them at this stage".
Professor Spector, though, says having seen the subsequent statistics this advice was wrong.
"I think sporting events should have been shut down at least a week earlier because they'll have caused increased suffering and death that wouldn't otherwise have occurred," he said.
The government replied that the blame could not be laid solely at the door of the sporting events.
"There are many factors that could influence the number of cases in a particular area, including population density, age, general health, and the position of an area on the pandemic curve," it said in a statement.
However, Edge Health, a firm which analyses data for Britain's National Health Service, has estimated the Champions League match was "linked to 41 additional deaths" at nearby hospitals between 25 and 35 days later, compared with similar hospital trusts that were used as a control.
Imperial College London and Oxford University have estimated Spain had around 640,000 positive coronavirus cases at the time of the match compared to 100,000 in Britain at that stage.
About 3,000 Spanish spectators were present when Jurgen Klopp's Liverpool lost to Atletico in the second-leg last 16 tie.
The match was the last major football fixture played in England before the coronavirus lockdown.
Liverpool could not call the match off unilaterally or have it played behind closed doors -- that decision would have been up to European football's governing body UEFA.
In response to the claim that the match had caused additional deaths, UEFA said: "Any decision taken by UEFA which led to matches being postponed or played behind closed doors was taken in close collaboration with, and based on decisions made by, the relevant national authorities in the respective host countries.
"UEFA did not receive any advice or request from local authorities to play this match behind closed doors."
UEFA said the British government had "stood by its position on the game".