An English landscape painting that has stirred a year-long controversy over whether it is a John Constable work was bought here today for $382,360 by the Yale Center for British Art and British Studies, the New Haven institution created and funded by Paul Mellon.

"The Young Waltonians--Stratford Mill--The Full-Scale Sketch," which is believed to have been completed by Constable in preparation for "The Young Waltonians" (1820), his famous painting of children fishing in a river, was the most eagerly anticipated picture up for bid today at Sotheby's all-day auction of Old Master and English paintings.

For the past year, Sotheby's experts have been working to gather enough evidence to present the six-foot-long landscape with full confidence that it is indeed by Constable.

Some dealers and a few experts have maintained that the picture remained unfinished in Constable's studio at his death and was completed by another hand to make it more saleable.

Nonetheless, the London art dealers Baskett & Day, who bid on behalf of Mellon, indicated they were confident that the painting--offered for sale from an unidentified Swiss collection--was, in its entirety, by the premier English landscape painter.

John Baskett, one of the gallery's senior partners, said in an interview after the sale he had been convinced by the favorable testimony of Graham Reynolds, a curator at the Victoria & Albert Museum here, who was preparing a catalogue raisonne' on Constable, and by the testimony of John Brieley, chief restorer at the Metropolitan Museum in New York, that the picture should be attributed to the artist.

"We are pleased that the center has been able to purchase the picture for such a low price," Baskett said. "It is clearly a great picture at a bargain price."

One indication of the uncertainty over the picture's authenticity was the estimated price set by the auction house, which had vacillated over the past year between $474,000 and $1,264,000.

"The Young Waltonians--Stratford Mill--The Full-Scale Sketch," named after Izaac Walton, the author of "The Compleat Angler," the fundamental British fishing guide for more than 100 years, was among the final pictures of a long auction. The sale attracted about 400 dealers and collectors to an unusually hot Bond Street sale room.

Among the items offered was "Landscape with Christ Appearing to His Disciplines at the Sea of Galilee," by Pieter Breughel, the elder. The star picture of the day, it failed to meet its reserve (minimum) price, even though bidding in the room reached $1,169,200.

The lusty and raucous "Mars and Venus Surprised by the Gods, taken from Ovid's Metamorphisis," by Joachim Antonisz Wpewael depicting the two gods embracing in love, was bought for $537,200 by a New York dealer.