More than 2,500 spectators are expected at Doncaster on Wednesday as the St Leger meeting starts amid uncertainty over new government rules.
It will be the first crowd at a British horse racing fixture in six months.
But the government said on Tuesday that social gatherings in England would reduce from a maximum of 30 to six people from Monday in response to rising numbers of Covid-19 cases.
Health Secretary Matt Hancock said more details would be announced later.
“The Prime Minister is going to set out more details on the consequences of new rules for six people gathering later today. We’ll set out what this means for some of these events that we were planning to do,” he told BBC Breakfast.
It is understood that Doncaster Racecourse has so far not received any request to change its plans, which are part of a government pilot scheme.
Prime Minister Boris Johnson is due to outline more details on the new rules at a Downing Street press conference at 16:00 BST, where he will be joined by Professor Chris Whitty, England’s chief medical officer, and Sir Patrick Vallance, the UK government’s chief scientific adviser.
Doncaster Central MP Rosie Winterton lobbied for the meeting to be held with a limited attendance as part of the government’s pilot sporting events, but the town’s mayor Ros Jones has said the risk of holding the fixture with crowds is “too great”.
Racing has been held behind closed doors since resuming on 1 June after a 10-week suspension because of the coronavirus pandemic.
Up to 3,640 people will be permitted entry on Wednesday, and the racecourse says more than 2,500 tickets have been sold. A limit of 6,202 is planned on the other days, including Saturday, when the Leger, the world’s oldest Classic race, is staged.
Racegoers must sign up to a code of conduct and they will be split into dedicated zones, with social distancing protocols in place.
More than 54,000 spectators attended the four-day meeting last year, including 27,000 on the Saturday,
A trial attendance of 5,000 people was due to take place on the fifth and final day of Glorious Goodwood last month, but was scrapped at the last minute after a spike in coronavirus cases across parts of Britain.