Christie's confirmed that an Asian client was the lone telephone bidder for the 1626-27 Rembrandt painting A Man in a Gorget and Cap, at £8.4 million.
The work was one of 11 paintings being sold by Pieter Dreesmann, the son of the late Dutch department-store heir Anton Dreesmann.
All the Dreesmann lots sold, raising £25.3 million.
Most had been acquired within the last 15 years from the Maastricht-based dealer, the late Robert Noortman.
The Constable was the most highly valued lot, at £20 million to £25 million, and was consigned by Baroness Carmen "Tita" Thyssen-Bornemisza, who became Baron Hans Thyssen-Bornemisza's fifth wife in 1985. It had been acquired by him at Sotheby's in 1990 for £10.8 million, then a record for any British painting sold at auction.
The work was guaranteed to sell courtesy of a third party "irrevocable bidder" that dealers identified as one of the auction house's Russian clients.
There were no other bidders and the unidentified guarantor was the buyer, Christie's said.
"There just aren't any private collectors for this kind of picture at the moment," the New York-based dealer Richard Feigen said.
"The Constable sold for the price of a second-tier Warhol. It's ridiculous."
The event raised £85.1 million with fees against an estimate of £61.8 million to £88.3 million, based on hammer prices. It surpassed the £68.4 million previous high for an Old Master auction at Christie's in December 2009.
Elsewhere, a Rodin bronze of The Kiss was snapped up at the Masterpiece London fair.
Masterpiece is billed as the UK capital's equivalent of the European Fine Art Fair in Maastricht, with the addition of luxury brands such as Rolls-Royces, Ruinart champagne and Vacheron Constantin watches.
More than 160 dealers are exhibiting in a temporary structure on the Chelsea Embankment.
Sladmore Gallery sold a bronze of The Kiss, cast during Rodin's lifetime, to a Swiss collector for US$2 million ($2.5 million). A Middle East client bought a pair of earrings by the Parisian jeweller JAR from Symbolic & Chase for US$500,000.
A hand-signed 1895 lithograph of Edvard Munch's The Scream - one of just 26 made - is being offered by the Oslo-based dealer Kaare Berntsen, for £1.7 million.
- Bloomberg