There are "serious questions" about why the Cheltenham Festival went ahead last month during the coronavirus outbreak, says the shadow sports minister.
There were 251,684 racegoers in attendance across the four days of the Festival, while 68,500 watched the Cheltenham Gold Cup on 13 March.
Catherine West told BBC Sport it was "worrying to hear several Cheltenham racegoers have contracted Covid-19".
The event took place just days before the government banned mass gatherings.
"Serious questions need to be asked about whether it was appropriate to have a mass gathering of tens of thousands of people while the rest of Europe were enacting social distancing and banning mass gatherings," West, who is Labour MP for Hornsey and Wood Green, added.
Cheltenham organisers introduced special hygiene measures for the four-day meeting, including extra hand-washing stations, and say they had followed clear guidance from the government and science experts.
Dr Sue Smith, senior racecourse medical officer for the Festival, said the hygiene standards were "of the highest level and all measures were taken in accordance with daily updates from Public Health England".
She added: "It's simply not possible to know how and where someone who has tested positive for Covid-19 has contracted it."
A spokesperson for Cheltenham Racecourse added: "The Festival concluded three weeks ago and went ahead under the clear and ongoing guidance from the government and its science experts throughout, like other popular sports events at Twickenham and Murrayfield, 10 Premier League matches and the Uefa Champions League at Anfield that same week."
A government spokesperson said: "It is our absolute priority to protect people's health and our advice on coronavirus is the result of direct, continuous consultation with medical experts. Cheltenham Festival was operating within clear Public Health England guidance at the time."