Stephen Yarborough was run over by the mobile starting stalls from which horses start running, operators RaceTech confirmed and police said he died at the scene.
The incident happened before the race due off at 15:30 BST and racing was abandoned for the day.
"We were deeply saddened to hear our friend and colleague Stephen Yarborough passed away today," RaceTech said.
John Bozza, RaceTech chief executive, added: "Our thoughts are very much with Stephen's family, close friends and his colleagues at this very sad time.
"Stephen was an experienced and popular member of the team; he will be hugely missed by everyone. We are doing all we can to support Stephen's family and the investigation at this stage."
Haydock general manager Jason Fildes said: "This is the worst possible news. I'm very upset for Stephen's family and friends and there is now real sadness among the whole team. We saw Stephen regularly in his role working for RaceTech. It's so tragic this has happened."
A Merseyside police statement said: "Emergency services were called at around 15.40 to reports that a starting stall coupled up to a tractor had been in collision with a male employee on the racecourse.
"The man, who was aged in his 50s, was pronounced dead at the scene. Next of kin have been informed."
The Professional Jockeys' Association said riders would wear black armbands at meetings on Saturday.
British Horseracing Authority (BHA) chief executive Nick Rust said: "British racing will come together in grief at this incident - we are one family and today we have lost one of our own."
A statement from the BHA said: "The stewards held an enquiry following a report from the starters that a member of the stalls team had been injured prior to race four.
"They interviewed both starters and the clerk of the course, who was representing the racecourse executive.
"The clerk of the course stated that the racecourse executive had taken the decision to abandon the remainder of the day's racing as the police had been informed of the accident and were due to arrive to conduct their enquiries."
Racing will go ahead as planned on Saturday, a spokesman for Haydock said.
Article Source: BBC News
Image courtesy BBC News image archive
Back to Hot off the Press