A 4,000-year-old Egyptian statue expected to raise about £6m has sold for £15.76m at Christie's of London.
Northampton Borough Council auctioned the Sekhemka limestone statue to help fund a £14m extension to Northampton Museum and Art Gallery.
However, Arts Council England had warned the council its museum could lose its accreditation status.
The Egyptian ambassador to Britain said the council should have handed the statue back if it did not want it.
Before the auction, Egyptian Ambassador Ahsraf Elkholy condemned the sale as an "an abuse to the Egyptian archaeology and the cultural property".
He said: "Our objection starts from this basic principle: how can a museum sell a piece in its collection when it should be on display to the public?"
The ambassador said: "We are concerned this piece may be moved into a private collection.
'Darkest cultural day'
"A museum should not be a store. Sekhemka belongs to Egypt and if Northampton Borough Council does not want it then it must be given back.
"It's not ethical that it will be sold for profit and also not acceptable. The council should have consulted with the Egyptian government."
Christie's said it would reveal details of the new owner later.
Protesters gathered outside Christie's before the sale said they wanted the statue to be returned to Egypt's Ministry of Antiquities.
Sue Edwards, from the Save Sekhemka Action Group, who travelled from Northampton to the auction, said: "This is the darkest cultural day in the town's history.
"The local authority has made a huge mistake but we will continue our fight to save Sekhemka."
Loss of Arts Council England accreditation would make the museum ineligible for a range of future grants and funding, however the leader of the council David Mackintosh said he did not see why this should happen.
Northampton Museum funding:
- £900,000 annual budget, according to Northampton Borough Council
- £166,000 granted in 2012 by Arts Council England for two projects and £69,000 granted in 2014 to digitise the museum's collection of 10,000 shoes online
- £615,000 Heritage Lottery Fund total grants for various projects
He said that having kept Arts Council England "informed of our actions and plans... we see no reason why we should not retain our accredited status".
The statue has not been on display for four years, and no-one had asked to see it in that time, he said.
"It's been in our ownership for over 100 years and it's never really been the centrepiece of our collection," he told BBC Look East.
"We want to expand our museum and to do that we need to raise the money."