Trainer Mark Johnston reacted with "desperate disappointment" to the announcement that the Queen's Vase at Royal Ascot will drop down in distance.
The historic staying race for three-year-olds has been elevated from Listed to Group Two class, but will from this summer be run over a mile and three-quarters instead of two miles.
Johnston has won the Queen's Vase seven times and criticised the move, which was approved by the European Pattern Committee as part of a remit to help "safeguard the future of the staying horse in Europe".
The Middleham handler told Press Association Sport: "After all the rhetoric about supporting and making a programme for stayers, my first reaction is desperate disappointment.
"In my eyes, the race is the three-year-old stayers' championship. Horses win that and then you can dream about the Ascot Gold Cup.
"To do that (drop down in trip) seems to be the opposite of what they are aiming to with the staying programme.
"It's not as if the race hasn't attracted great horses and great winners, especially in recent times."
EPC chairman Brian Kavanagh said the decision to make the Queen's Vase a less demanding test of stamina was made to target "a greater number of quality young horses".
By contrast to Johnston, Newmarket trainer William Haggas welcomed the news.
He said: "The existing trip of two miles is a very stiff test for a three-year-old in mid-June, whereas the slightly reduced distance will see the race appeal to a wider pool of young horses with potential to be high-class stayers of the future.
"This is the key three-year-old staying race in the first half of the season and there is good reason to make it a real target, rather than an afterthought for horses felt to perhaps not be good enough to win over 12 furlongs."
The Qatar Goodwood Cup on August 1 has also been upgraded to Group One status in the drive to enhance the European Pattern programme for stayers.
Trainer Michael Bell has won the two-mile race for the last two seasons with Big Orange and said: "I think this is a very good thing for the staying division.
"It fits in nicely, as stayers are short of a Group One over two miles at that time of the year.
"It may make it a bit tougher for Big Orange to notch a hat-trick, but it's very good news."
The Goodwood Cup and Queen's Vase will also receive extra prize-money, while the EPC said that no European Pattern race over 13 furlongs or farther will downgraded before 2022 - unless a racecourse chooses to do so voluntarily.
Ruth Quinn, director of international racing and racing development for the British Horseracing Authority, believes the changes will have a long-term galvanising impact on stayers across Europe.
She said: "The project is about building for the future and this will take patience, commitment, and determination.
"We believe that in time our agreed strategy will help make the stayer a more desirable asset in British and European racing."
Respected Newmarket trainer John Gosden said the proposals "can only be of benefit to racing and to the breed".