Satellite data shows the Conger ice shelf, which had an approximate surface area of 1,200 sq km, has collapsed, say scientists.
Temperatures were 40ºC higher than usual in the area due to an atmospheric river that had trapped heat over the continent, raising the temperature to -11.8ºC.
Ice shelves slow the flow of inland ice into the ocean, helping to prevent the sea level rising.
Dr Catherine Colello Walker, an earth and planetary scientist at Nasa and the Woods Hole Oceanographic Institution, said the shelf probably hit its tipping point as result of the temperature rise.
“It is one of the most significant collapse events anywhere in Antarctica since the early 2000s when the Larsen B ice shelf disintegrated,” she told The Guardian.
Professor Matt King, who leads the Australian Centre for Excellence in Antarctic Science, said it could be a sign of things to come.
“Taste of more nasty surprises to come from East Antarctica?” he tweeted.
“We will see massive ice shelves — way bigger than this one — break up. And those will hold back a lot of ice — enough to seriously drive up global sea levels,” he told The Guardian.