Seven-time champion jockey John Francome believes moving the King George VI Chase from Kempton Park to Sandown Park would be a "huge mistake".
Kempton Park could close to make way for 3,000 homes as part of plans to raise £500m to invest in horse racing.
Should the proposal go ahead, Kempton's famous King George VI Chase would move to Sandown, located six miles away.
"A lot of horses that have won the King George at Kempton wouldn't win at Sandown," Francome said.
He told BBC Radio 5 live's Sportsweek: "At Sandown, Cheltenham and quite a few other tracks, you race downhill. Horses get to free-wheel, but there's no free-wheeling at Kempton and it's a really, really good test.
"It's level, it's fair and you've got to be able to lay up with the pace the whole way - in many ways, it's the fairest test there is. I just think it will be a huge mistake."
Ten-time champion trainer Paul Nicholls has had nine winners of the King George VI Chase, including five with Kauto Star, and thinks a move to Ascot would be a better option.
"While I think all my King George winners would also have won at Sandown, because they stayed so well, I'm not in favour of the proposal to switch the race there," said Nicholls in his Betfair column.
"Given the choice, I'd move it to Ascot - a much more suitable venue from a racing point of view as it is an outstanding jumps track with excellent facilities and the necessary infrastructure to accommodate a large crowd.
"Just imagine more than 30,000 jumping enthusiasts at Ascot on Boxing Day for the most important jumps race in mid-winter."
London "a tough market"
Kempton course owner, the Jockey Club, says the proposal, with Redrow Homes, to close the course is "for the long-term good of British racing".
Jockey Club chief executive Simon Bazalgette said: "It's a very tough market to get people to go racing in London.
"We can really focus on Sandown, which, of course, everyone loves and is one of the best-viewing courses in the country.
"We can get racing back to being a core part of London life for millions of people.
Racing will continue at Kempton until at least 2021, and the redevelopment will only proceed if more than £100m is raised from the sale and an all-weather circuit - likely to be located at Newmarket - is given the go-ahead.
"I've had so many magical days at Kempton, some of the best of my career with lots of success," added Nicholls.
"Obviously I dread the thought of the doors at Kempton closing for the final time and hope it doesn't happen."