The mystery behind the sale of a £49.5m masterpiece

There was a breathless hush as the auctioneer opened the bidding at £3m, then pushed it quickly to £3.2m and £3.8m. But for the dealers in the packed room, their eyes on the hottest Old Master to appear in years, the offers were too pedestrian. "£6m!" one shouted, and then everything seemed to go crazy.

On they went in increments of £1m until, at more than £49m, Rubens' Massacre of the Innocents became the most valuable painting on earth. And the sale took less than 10 minutes.

"It all seemed to go terribly quickly. I was quite unnerved by that," said Sam Fogg, the dealer who bought the painting for a private collector at Sotheby's in London on Wednesday.

"It felt no longer than a couple of minutes at most, and I didn't even come in until it got to £26m. I remember that there was a dealer in the front row who started bidding at £16m or £17m and he continued until £41m. Then, for the last £4m there was a telephone bidder. At the end, the auctioneer was very slow to finally bring the hammer down.

"I was simply delighted that my client was able to acquire it. It really is the most wonderful Old Master painting it has been possible to buy for many, many years. The chance of something like it coming up again is very remote."

The final price of the painting, with buyer's premium, was £49,506,650, beating the previous record of £49,005,059 paid in 1990 by a Japanese businessman for Vincent van Gogh's Portrait of Doctor Gachet. The Massacre of the Innocents depicts the slaughter of newborn boys ordered by King Herod because of his fears of being usurped by the Messiah.

Asked whether his client was likely to lend the picture for public display, Mr Fogg said: "I do think it is likely to be seen by the public again."

The story behind the sale of the painting is as astonishing as the bidding. Until late last year, the work was thought to be by the Flemish painter Jan van den Hoecke, a student of Peter Paul Rubens who became one of the most popular painters at the Viennese court. It was the property of an Austrian woman whose father bought it in 1920 from a dealer in Vienna. He died in 1923.

During the 1930s, the Massacre of the Innocents was exhibited by a dealer in Dresden before being hung in the vendor's husband's office in Vienna until the spring of 1945. It had been moved to the safety of Salzburg when the office building was destroyed in an Allied bombing raid. After the war, it was moved around again – once to the walls of a furniture store – before being sent on loan in 1973 to Stift Reichersberg, a monastery in Austria.

Late last year the owner's family approached Judith Niessen, one of Sotheby's experts in Amsterdam, with a view to selling it. She immediately contacted George Gordon, the auction house's most senior authority on Old Masters. "She e-mailed me a photograph and my first reaction was that it was an extremely good picture and we had better get over there and see it," Mr Gordon said. "The painting was hanging, high up on the wall in the monks' quarters. We couldn't see it properly but I remember my reaction was that this was a picture of great quality.

"When we got it back to London in good light at ground level it became pretty obvious what it was ... not a Van den Hoecke, but a Rubens."

Mr Gordon showed the painting to Rubens scholars, tracked its provenance, studied letters and inventories of dealers dating back to the start of the 18th century and even had the rings counted from a section of the frame. "It was terribly exciting," he said. "There was a hell of a lot of detective work, but it was worth it. I doubt such a painting will come my way again."

The sale capped an incredible month for Sotheby's and indicated the buoyancy of the art market. Last month, an Impressionist and Modern Art sale fetched £38.8m while this week a sculpture attributed to Gian Lorenzo Bernini, estimated at £180,000, fetched more than £2m.