Saturday is expected to be the most lucrative day in the UK horseracing calendar, with £200m likely to be staked on the Grand National.
But to the anger of the industry, huge numbers of bets are likely to escape the tax that funds the sport. These bets will come through smartphones, tablets and computers and be routed to offshore hubs.
William Hill said it was likely to take "in excess of a million" bets on its mobile app, with more than 10,000 bets a minute coming through smartphones and computers in the run-up to the race.
"Forty per cent of digital bets do not contribute to British racing because they go offshore," said Nick Rust, the former Ladbrokes executive who now runs the British Horseracing Authority, the sport's regulator.
He said the big four bookmakers, William Hill, Ladbrokes, Gala Coral and Betfred, did pay a "voluntary contribution" but said it did not fully reflect the profits they made from online betting. Others, such as Paddy Power and Sky Bet, did not pay a contribution.
"That is not something that the average member of the public is aware of. If they understood their bets do not support the 85,000 people who work in horseracing, the people who wake up at 5.30am to look after the horses, they might be horrified."
The feud between racing authorities and the bookmakers over how to support the sport has been raging for decades. A tax on gambling, called the Horserace Betting Levy, was introduced in the 1960s to compensate racecourses for the money they lost when people place bets off-course, in betting shops.
The levy remains in place, charging bookmakers 10.75 per cent of their gross profits for taking off-course bets.
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But the racing industry, which also funds itself by selling media rights, believes that between £20m and £35m is being dodged as bets migrate offshore. "We do not want to gouge the bookmakers," Mr Rust said. "We want to close the net."
The bookmakers have strongly resisted any change to the status quo. Despite the glamour of big events such as the Grand National, the sport is becoming less important to them.