A giant block of ice measuring 260 sq km (100 sq miles) has broken off a glacier in Greenland, according to researchers at a US university.
The slab of ice separated from the Petermann Glacier, on the north-west coast of Greenland.
It is the largest Arctic iceberg to calve since 1962, said Prof Andreas Muenchow of the University of Delaware.
The ice could become frozen in place over winter or escape into the waters between Greenland and Canada.
If the iceberg moves south, it could interfere with shipping, Prof Muenchow said.
Cracks in the Petermann Glacier had been observed last year and it was expected that an iceberg would calve from it soon.
The glacier is 1,000 km (620 miles) south of the North Pole.
A researcher at the Canadian Ice Service detected the calving from Nasa satellite images taken early on Thursday, the professor said.
The images showed that Petermann Glacier lost about one-quarter of its 70km-long (43-mile) floating ice shelf.
There was enough fresh water locked up in the ice island to "keep all US public tap water flowing for 120 days," said Prof Muenchow.
He said it was not clear if the event was due to global warming.
Patrick Lockerby, a UK engineer with a background in material science, told the BBC he had predicted the calve on 22 July, posting images on the science2.0 website.
"I was watching the floating ice tongue wedged between two walls of a fjord for three quarters if its length with the last part at the outlet end wedged by sea ice. I thought once the sea ice was gone, the pressure would be too great and the tongue would calve."
He said there could be a beneficial outcome if the calving drifts to block the Nares Strait and effectively prevents the loss of more ice from the Lincoln Sea.
The first six months of 2010 have been the hottest on record globally, scientists have said.