The two-foot wide crack, which is 45 metres long, was witnessed by professional photographer Doug Chalk on Friday near West Bay.
Describing the crack, he said it looked "pretty awful".
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The crack has been examined by Jurassic Coast earth science manager Richard Edmonds, who said a disaster could be inevitable.
He said, "It's clearly going to fall off - I just can't tell you when. It's quite concerning but it's part of the nature of this coastline."
It is understood that the crack could be related to heavy rainfall which was seen across 2012 and 2013.
The crack is also being examined by Dorset County Council, which has installed temporary warning signs.
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A spokeswoman told the BBC, "Advice to coast users is, as always, for people to stay away from the edges and base of cliffs, to observe warning signs, whether temporary or permanent, to stay on official paths, and to be aware of their surroundings."
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