Satellite images suggest that some of the adjoining ice shelf will collapse in the next few days or weeks. The Wilkins Ice Shelf has been stable for most of the last century, but began retreating in the 1990s. Six ice shelves in the same part of the continent have already been lost, says the British Antarctic Survey (BAS).
Professor David Vaughan of BAS said: "Wilkins is the largest ice shelf on the Antarctic Peninsula yet to be threatened.
"I didn't expect to see things happen this quickly. The ice shelf is hanging by a thread - we'll know in the next few days or weeks what its fate will be."
Jim Elliott, who was on board the plane, said he had never seen anything like it before.
He said: "We flew along the main crack and observed the sheer scale of movement from the breakage.
"Big hefty chunks of ice, the size of small houses, look as though they've been thrown around like rubble - it's like an explosion."
A 41-by-2.5km (25-by-1.6 mile) berg has already broken away, and a large part of the Wilkins Ice Shelf is now protected only by a thin strip of ice spanning two islands.
Since an ice shelf is a floating platform of ice, its loss will have no impact on sea level. But scientists say it heightens concerns over the impact of climate change on this part of Antarctica.
Professor Vaughan predicted in 1993 that the northern part of the Wilkins Ice Shelf would be lost within 30 years if climate warming continued. But he said it is happening more quickly than he expected.
He told BBC News: "What we're actually seeing is a chunk of the ice shelf drop off in a way that suggests it is not just a normal part of iceberg formation.
"This is not a sea level rise issue, but is yet another indication of climate change in the Antarctic Peninsula and how it is affecting the environment."
Scientists say the Antarctic Peninsula, which juts out into the Southern Ocean towards the tip of South America, has experienced unprecedented warming over the last 50 years.Several ice shelves have retreated in the past 30 years - six of them collapsing completely.