A iceberg measuring 1,270 square kilometres - almost the size of Greater London - broke away from the Antarctic ice shelf, the British Antarctic Survey said on Friday.
The research body said the iceberg broke from the 150-metre-thick Brunt Ice Shelf in a process called calving.
This happened almost a decade after scientists first reported cracks in the shelf.
One crack in the ice widened by several hundred metres on Friday morning before the iceberg broke off completely.
Britain's nearby Halley VI Research Station monitors the state of the vast floating ice shelf daily.
"Our teams at BAS have been prepared for the calving of an iceberg from Brunt Ice Shelf for years," said BAS director Jane Francis.
The mobile research base relocated inland for safety reasons in 2016-2017 as cracks in the ice threatened to cut it off.
"That was a wise decision," said Simon Garrod, BAS director of operations.
The glaciologists said the latest event was unlikely to affect the station's location.
The base's team of 12 departed earlier this month, as they leave the base uninhabited in winter because of the unpredictable conditions.
While they are away, data from GPS instruments at the site goes to a centre in Cambridge, eastern England, for analysis.
Icebergs naturally break off from Antarctica into the ocean in a process accelerated by climate change.
The BAS said in this case, there is "no evidence that climate change has played a significant role".
"Over coming weeks or months, the iceberg may move away; or it could run aground and remain close to Brunt Ice Shelf," said Ms Francis.
The British Antarctic Survey is a world leader in environmental research in the region.