A massive ice island has broken off from one of Greenland's two main glaciers, in the biggest such event in the Arctic in nearly 50 years.

The new ice island, which broke off on Thursday, will enter the Nares Strait, about 1,000km south of the North Pole between Greenland and Canada.

The ice island has an area of 260sq.km and a thickness up to 190m in places.

Andreas Muenchow, professor of ocean science and engineering at the University of Delaware, said they had expected an ice chunk to break off from the Petermann Glacier.

Prof Muenchow said it was expected because it had been growing in size for seven or eight years, but he did not expect it to be so large.

'The freshwater stored in this ice island could keep the Delaware or Hudson Rivers flowing for more than two years,' said Prof Muenchow.

'It could also keep all US public tap water flowing for 120 days.'

He said it was hard to judge whether the event occurred due to global warming because records on the sea water around the glacier have only been kept since 2003.

The flow of sea water below the glaciers is one of the main causes of ice calvings off Greenland.

'Nobody can claim this was caused by global warming. On the other hand nobody can claim that it wasn't,' Mr Muenchow said.

Scientists have said the first six months of 2010 have been the hottest globally on record.

The El Nino weather pattern has contributed to higher temperatures, but many scientists say elevated levels of man-made greenhouse gases are pushing temperatures higher.

The last time such a large ice island formed was in 1962 when the Ward Hunt Ice Shelf calved an island.

Smaller pieces of that chunk became lodged between real islands inside Nares Strait.