Britain's bookies are bracing themselves for possible £40m hit on this weekend's Epsom Derby.

And incredibly it isn't last year's winning jockey Frankie Dettori or this year's Derby favourite Casual Conquest that could cause them the biggest headache - it's the prospect of the 1000/1 outsider Maidstone Mixture winning that would cost bookies dearly.

Hills spokesman David Hood said: "History demonstrates that on these big races, an average betting shop will take £2 to £3 in bets for even the most unlikely winners.

"With 10,000 betting shops in the UK and Ireland and those in the Kent area probably fielding £10-£20 for him at such a big price, he could easily cost bookies well over £40m."

Owned by eccentric property tycoon Fergus Wilson, the grey colt only managed to win a race on his tenth start - and that was hurdle race in France - but Maidstone Mixture will be running on Saturday in the Derby in an attempt to 'live the dream' for his 60-year-old owner.

Added Hood: "Every punter in the land will have at least have a cursory glance at Maidstone Mixture, simply because of his enormous odds, and plenty will be prepared to chuck away a quid or two........ 'just in case'.

Last year Frankie was the darling of the punters on the 7/4 favourite Authorized, but this year's Derby is one of the most open for years, and whilst Frankie will have his backers again (on 16/1 chance Rio De La Plata) ironically it's the rank outsider that could hurt us most."

This weekend sparks another betting bonanza for punters, with Hills predicting more than £150million turnover in all.

In addition to the Derby, punters have the start of the Euro 2008, the opening weekend of Big Brother, the Canadian Grand Prix, and the French Open finals for tennis.