One of horse racing's most revered jumpers will be banned from competition in Australia at the end of the 2015-2016 season.

Steeplechaser Bashboy will have to be retired at the end of July - or be sent overseas to race - under a new Australian rule that says thoroughbreds cannot compete past 12 years of age.

The rule means the three-time Grand National Steeplechase winner won't be able to defend his crown.

This year's Grand National will be run on August 21.

Racing fans have vented their anger over Bashboy's fate on social media and the veteran started trending on Twitter following confirmation his racing days were numbered.

A Racing Victoria statement said the governing body would not try to have the rule changed.

"The RV Board considered a proposed amendment that would allow jumpers that turn 13 years of age in a calendar year to be exempted from retirement until the end of that year's jumps season on 30 September," the statement read.

"In the interest of horse and participants' welfare, the Board determined that an amendment to the current rules would present a greater safety risk to the horse, their rider and other participants and, as such, will not seek to make an amendment."

The Ciaron Maher-trained Bashboy will also be ineligible to contest the Crisp Steeplechase, a race he has won four times.

Co-owner Ian Macdonald said he was frustrated by a lack of consultation from Racing Australia, claiming the age limit was random.

"They've in effect turned their backs on promoting jumps racing by actively supporting the retirement of the current Australian champion, which is totally ridiculous," Macdonald told Melbourne radio station RSN.

"No one was consulted on the rule - it seems they picked an arbitrary age it is deemed to retire a horse.

"It's totally arbitrary - why not make it 12 or 10 (years of age)."

Macdonald said an overseas campaign was an option for the popular horse who has won 18 races and earned almost $1.1 million in stakes.