Sir Anthony McCoy described the prospect of Kempton being redeveloped for houses as "terrible" and expressed his doubts about Sandown being able to fill the breach.
Jockey Club Racecourses, who own both tracks, announced on Tuesday that Kempton, home of the historic King George VI Chase, could be closed for housing redevelopment "from 2021 at the earliest".
The Jockey Club hopes to invest £500million into the sport in a 10-year plan, with nearby Sandown receiving some significant upgrades as well as being the proposed new home for the King George and a clutch of other big Kempton races.
However, McCoy has expressed his doubts about whether the Sandown surface can take any additional racing.
He told The Times: "There will be a lot of people who will be disappointed. It is terrible we will be losing a grade one jumps track and we can't afford that.
"For grade one horses, indeed any horses, I am not sure Sandown has enough ground to cope with the fixtures it already has.
"It is not wide enough. At this time of year trying to get good, safe ground is paramount. The more racing, the less unused ground you get.
"There is no width around the bottom bend at Sandown, it is very narrow and there is nowhere to expand."
Meanwhile, Spelthorne Borough Council leader Ian Harvey has reiterated his opposition to the idea of redeveloping Kempton.
The Jockey Club has paired up with Redrow Homes as its development partner and after Cllr Harvey called on the firm last year to "desist from unwanted desecration of desperately needed green belt space", his view remains the same.
He told www.getsurrey.co.uk: "I am fully aware of the views of local residents and have consistently made clear my strong opposition to development of this site and my determination to protect the green belt in Spelthorne.
"I made my position absolutely clear in an open letter which I wrote to Redrow homes in April. Nothing has changed. "