A vast iceberg, expected to be one of the biggest ever recorded, with an area almost the size of Prince Edward Island, is poised to break off Antarctica.

A rift, slowly developing across the Larsen C ice shelf on the Antarctic Peninsula in recent years, expanded abruptly last month, growing by about 18 kilometres. It is now more than 80 kilometres long with just 20 kilometres left before it snaps, scientists said.

"The Larsen C Ice shelf in Antarctica is primed to shed an area of more than 5,000 square kilometres following further substantial rift growth," scientists at Project Midas at the University of Swansea in Wales said in a statement.

A map illustrates the extension of the rift. (Project MIDAS/Martin O'Leary)

The iceberg "will fundamentally change the landscape of the Antarctic Peninsula" and could herald a wider break-up of the Larsen C ice shelf, the statement said.

Ice shelves are areas of ice floating on the sea, several hundred metres thick, at the end of glaciers. Scientists fear the loss of ice shelves around the frozen continent will allow glaciers inland to slide faster towards the sea as temperatures rise because of global warming, raising world sea levels.

Several ice shelves have cracked up around northern parts of Antarctica in recent years, including the Larsen B that disintegrated in 2002.