The discovery, by scientists from Bristol University and the British Antarctic Survey in collaboration with US colleagues, is reported this week (September 24) in the international journal Science.

The stability of the West Antarctic Ice Sheet has been hotly debated since the 1960s because of its potential to raise global sea level by around 5 m over several centuries.

The potential impacts of a major change in the West Antarctic ice sheet are severe – sea level rise will be fantastically expensive for developed nations with coastal cities and dire for poor populations in low-lying coastal areas.

Lead author Prof Martin Siegert of Bristol University said, 'There is a great deal of speculation that global warming may cause sea levels to rise due to the melting of ice sheets.

Until now, scientific observations suggested that change to the West Antarctic Ice Sheet would be restricted to the edges implying that large-scale instability of the ice sheet is unlikely.

This new discovery deep within the ice means that we need to re-think our current assessment of the risk of collapse of this ice sheet.'

The structure - a distinctive fold in the ice, 800m deep by 50 km long - was detected using ice-penetrating radar. Ice sheets normally consist of flat layers of ice, so finding this huge fold was a complete surprise.

Its presence suggests that a few thousand years ago surface ice at the centre of the ice sheet was moving rapidly and being 'drawn down' towards the bottom of the ice sheet.

More recently the rate of the ice flow has changed from fast to slow. The direction of flow has also changed. The most likely explanation for these changes is the 'switching-off' of a large ice stream at the margin of the ice sheet several centuries ago.

These changes imply that the centre of the ice sheet is more mobile than scientists previously realised, requiring them to rethink existing models.

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