Robert Sangster, the British bookmaker with the penchant for expensive thoroughbreds, tied his own world record yesterday by bidding $4.25 million for a son of fabled sire Northern Dancer at the Keeneland July Selected Yearling Sale in Lexington, Ky.

And Sangster, whose partners in the purchase included Greek shipping tycoon Stavros Niarchos, hinted that, before the end of the opening-day session, another horse would stand alone as the most expensive ever to be sold at public auction.

He said he did not consider the Northern Dancer-Desert Vixen colt, consigned by North Ridge Farm, to be the premier offering of the Keeneland sale.

However, the next highest bid of the evening was $2.7 million for a Mr. Prospector colt with ties to Northern Dancer through his dam, Ocean's Answer. Ocean's Answer is by Northern Answer, a son of Northern Dancer.

Earlier yesterday Sheik Mohammed Al Maktoum of Dubai paid $4.1 million for a son of Nijinsky II and a record $2.5 million to bring a filly by Northern Dancer to his English thoroughbred farm.

The record-setting filly, a half-sister to Canadian horse of the year Glorious Song through her dam, Ballade, was consigned by Windfields Farm in Maryland, where Northern Dancer stands at stud.