A long-running rift in the Larson C ice shelf grew suddenly in December and now just 20km of ice is keeping the 5000 sq km piece from floating away.
Larsen C is the most northern major ice shelf in Antarctica.
Researchers based in Swansea say the loss of a piece a quarter of the size of Wales will leave the whole shelf vulnerable to future break-up.
Larsen C is about 350m thick and floats on the seas at the edge of West Antarctica, holding back the flow of glaciers that feed into it.
