Coastguards are urging everyone to keep away from the coast near Whitby after part of the Cleveland Way collapsed into the sea.
They fear there could be further collapses as the coast along the popular long distance walking route is now potentially very unstable.
This afternoon, people walking towards along the dangerous stretch were being turned back before they reached it.
North Yorkshire Police have put up a cordon around the affected area and permanent signs and fencing will start to be erected tomorrow.
The alarm was sounded mid-afternoon when a walker walking along the Cleveland Way saw that the route ahead of them had disappeared and alerted the Royal National Lifeboat Institution, who then alerted Humber Coastguard who sent teams from Staithe and Whitby to inspect the scene.
Speaking on Radio York, Humber Coastguard watch manager Mike Puplett said: "Because there have been two landslips, the surrounding area may well be liable to go as well.
"There is not a great deal we can do about the effects, indeed all we can do is look at the aftermath and try to make it safe for people."
He urged everyone not to cross the cordon or go to the cliff edge further along and try and look down at the collapsed cliff.
The affected area is near Saltwick Bay which is between Whitby and Robin Hoods Bay. It is remote and anyone who gets into difficulty would have problems summoning help.
The North Yorkshire coast is prone to landslips caused by the effect of sea waves. The length of land affected has yet to be measured but the landslip was said to be "substantial".
Scarborough Borough Council is also involved in assessing the impact and taking safety precautions.
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