Warwick and Warwick auctioneers had expected the stamp, one of a sheet of one hundred discovered 90 years ago, to fetch 150,000 pounds when it went under the hammer. The final price includes buyer's premium.

Although it exceeded pre-sale estimates, two other "inverted Jennys" were sold in 2007 for $825,000 and $850,000 before the buyer's premium was added, Warwick and Warwick said on its website.

"It's very difficult to make comparisons," said auctioneer Colin Such. "Those sold for over $800,000 were in exceptional condition while this had damage to it."

The 24-cent airmail stamp features a Curtiss JN-4 Jenny aircraft in flight inadvertently printed upside down.