Images taken by its Envisat satellite show that the so-called Larsen B ice shelf decreased from 11,512 square kilometres (4,373 square miles) in 1995, an area about the size of the Gulf state of Qatar, to only 1,670 sq km (634 miles) today.
Larsen B is one of three ice shelves that run from north to south along the eastern side of the peninsula, the tongue of land that projects towards South America.
From 1995 to 2002, Larsen B experienced several calving events in which parts of the shelf broke away. It had a major breakup in 2002 when half of the remainder disintegrated.
Larsen A broke up in January 1995.
"Larsen C so far has been stable in area, but satellite observations have shown thinning and an increasing duration of melt events in summer," the agency said in a press release.
Ice shelves are thick floating mats of ice, attached to the shore, that are created by the runoff into the sea from glaciers.
Scientists say they are extremely sensitive to changes in atmospheric temperature and can be hollowed out from below by warmer ocean currents.
The northern Antarctic peninsula has been subject to atmospheric warming about 2.5 degrees Celsius (4.5 degrees Fahrenheit) over the last 50 years, a figure that is several times greater than the global average.
Ice shelves are not the same as ice sheets, the vast blanket of frozen water that covers Antarctica.
If these melted, even partially, they would drive up sea levels, threatening small island states and coastal cities. But the scientific evidence is that the icesheets so far are stable.
"These observations are very relevant for measuring the future behaviour of the much larger ice masses of West Antarctica if warming spreads further south," ESA quoted Helmut Rott, a professor at the University of Innsbruck in Austria, as saying.