The Treskilling Yellow had been expected to fetch up to £1.7million.
But the exact price paid was not disclosed when the little Swedish rectangular treasure, the world's most valuable object by weight, was sold to a group of mystery buyers in a telephone auction in Geneva on Saturday.
'It is still worth more than any other single stamp including the even older 2 Penny Post Office Mauritius Blue that sold for £970,000 in 1993,' said auctioneer Mr David Feldman.
He said the Treskilling Yellow was last displayed in public a week ago at the London 2010 Festival of Stamps.
He added that both the price and identity of the buyers, who took part in the telephone auction against a single rival bidder, would likely become public knowledge eventually.
Mr Feldman said yesterday's auction 'happened very quickly because the buyers clearly knew how much they were prepared to pay'.
The economic downturn has made people more careful about bidding for such pricey items, he said.
At the same time, stamps are increasingly being seen as investment opportunities, he added.
'People are looking for investments they can hold in their hands,' he said.
The Treskilling Yellow has changed hands at least once since 1996, but little is known about the last owner except that he or she had to sell the stamp because it had been put up as financial collateral.
The first collector to own the stamp is said to have been a Swedish schoolboy, who found it in 1885 among a pile of letters left by his grandparents.
The Treskilling Yellow is 'one of about a half dozen highly notorious stamps in the world,' according to noted U.S. stamp expert Robert Odenweller.
He added: 'The people who run around with that kind of change in their pockets have their own ideas about publicity.'
The 1856 British Guiana 1 cent Magenta is the best-known of all unique stamps, but has been locked away in a vault since 1980 when it was bought for nearly one million US dollars by chemicals fortune heir John du Pont.
He is currently serving a 13 to 30-year sentence for third degree murder.
According to www.dailymail.co.uk