Four race meetings were cancelled today as jockeys, trainers and officials united against Chancellor Rachel Reeves’ plans.
The dispute centres on Treasury moves to harmonise online gambling taxation.
Bookmakers currently pay 15 per cent on racing bets, compared with 21 per cent on online casinos.
Proposed plans would see racing bets fall in line with that of online casinos.
The British Horse Racing Authority warns the change would strip £330million from the sport over five years.
It also forecasts more than 2,750 job losses in the first 12 months.
British Horse Racing Authority (BHA) chief executive Brant Dunshea said: "This is an existential threat for the sport."
He added: "Racecourses are the focal point of towns and communities across the country. The training facilities, the studs and the breeding yards all play a central role in those communities."
Mr Dunshea said: "British racing is woven into the national fabric. For all the social benefits we bring, we should be treated differently."
Shutting down is the sport raising the stakes with the government in the latest battleground over the budget.
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Jockey Tom Marquand told Sky News that the shutdown was a "big deal" for riders.
Mr Marquand said: "We are self-employed. If you take away prize money and funding, it immediately reduces the opportunities available. It makes it much harder to sustain a living."
He warned: "If you pull away the base, everything above crumbles — and racing is no different."
Assistant trainer Sarah Guest said: "Smaller yards are struggling to keep owners engaged. Without them, we cannot keep going."
Horse racing contributes £4.1 billion to the economy and supports more than 85,000 jobs across Britain | GETTYMs Guest said: "The costs are going up all the time. Owners are asking daily whether they can justify staying in the sport. Many cannot afford it, and that hits us directly."
Trainer John Berry, a former mayor of Newmarket, said: "I have made a profit just once in 30 years."
Mr Berry said: "British prize money is very poor compared with international levels. The sport is kept afloat by overseas investment and goodwill. Any extra pressure will drive more owners away."
The Treasury has not commented while Ms Reeves prepares her November 26 budget.
The BHA forecasts more than 2,750 job losses in the first 12 months.
| GETTYExchequer Secretary Dan Tomlinson said: "We are working closely with the industry to understand any potential impacts."
When racing shuts down on Wednesday - with four scheduled events rescheduled - the sport will converge on Westminster to lobby for the tax hike to be scrapped.
With racetracks silent and voices across the industry united, the sector has sent its strongest warning yet.