The decision to allow the Cheltenham Festival and a Liverpool Champions League game to go ahead in March 'caused increased suffering and death', according to the scientist leading the UK's largest Covid-19 tracking project.
Speaking to the BBC, Professor Tim Spector said that the rates of coronavirus cases reported locally 'increased several-fold' after the events took place.
Despite coming under pressure to cancel or postpone the meeting, the Cheltenham Festival ran for four days from March 10 to March 13.
In football, Liverpool hosted Spanish side Atletico Madrid in the Champions League on March 11.
The same night that Anfield was sold out for the European tie, PSG took on Borussia Dortmund in Paris - although that game was played behind closed doors.
Prior to these sporting events taking place in the UK, Culture Secretary Oliver Dowden told the BBC that there was no reason for them to be cancelled.
"There's no reason for people not to attend such events or to cancel them at this stage."
Professor Spector, from King's College London, has said that 'people will have probably died prematurely' because of the decision for the sporting events to go ahead.
In the Six Nations, Ireland vs Italy was postponed, having been scheduled to take place on March 7, while England vs Wales proceeded at Twickenham on the same day. Prime Minister Boris Johnson was in attendance for the England win.
INM journalist Melanie Finn was at the Cheltenham Festival to cover the action, but returned home due to safety concerns, with many race-goers not heeding warnings around coronavirus precautions.
"When we flew out of Dublin Airport it was literally like a ghost town," she said.
"It [Cheltenham] was like the last days of the Roman Empire, and I think there was a little bit of a sense that if it was open, by God they were going to party."