It began as a low rumble on Tuesday night, but soon giant chunks of land "the size of cars" were cascading into the sea off Dorset. By yesterday morning, a 400m section of the World Heritage Jurassic Coast between Lyme Regis and Charmouth had disappeared, in what has been described as the biggest landslide Britain has seen in a century.

When panicked residents of Lyme Regis first heard the noise at around 8pm they called the coastguard, who spent hours trying to make the area safe. When they left later that night it was "still rumbling".

The 150m-high cliffs are part of the Jurassic Coast, a 95-mile stretch of protected coastline between Dorset and east Devon. It was England's first Unesco World Heritage Site, because the rocks contained in the cliff faces include fossils dating back 185 million years.

When landslides on this scale occur, they often uncover the fossils for which the region is famous. However, West Dorset District Council was warning fossil enthusiasts to stay away from the beach until the cliff was stable. The council's planning and environment director David Evans said: "I would strongly advise people to keep away from the landslip for their own safety. It is not in a fossil area and is extremely muddy."

Simon Palmer, Portland Coastguard watch assistant, said protecting the public from the unstable cliffs was still a chief concern. "The problem is that keen fossilers will descend because it will turn up a lot of new fossils," he said. "Our biggest concern is the danger to members of the public putting themselves in danger by getting underneath these potentially hazardous lumps of rocks." Dorset police described the slip as the "biggest in 100 years", but geologists said there was a similarly large slide in 1986.

Richard Edmonds, earth science manager for the Jurassic Coast World Heritage Team, said: "The last time we had one this big was in 1986, but this is pretty spectacular." Although this stretch of coastline is famous for its erosion - a process that has uncovered its magnificent fossils - it is very rare to see such dramatic landslips occur in one go. Mr Edmonds added: "This coast has landslides all the time, but this was really big - 400 metres of the cliff has burst out on to the beach."

He described his excitement at hearing the news, as he had hoped it would uncover exciting new fossils. "I was up from five o'clock this morning and I didn't find anything, it was really disappointing," he said. "There will be fossils, but it might not be until next winter that we can see them." Despite warnings from the police, the coastguard and the council, many fossil collectors had gathered at the site yesterday morning.

The World Heritage team recommended that the fossil hunting was left to the professionals. "Collectors were poking through the rubble this morning, but it's not safe yet", said Mr Edmonds. "There are still lumps the size of cars rolling down the cliff face. It's still fairly hairy."

He said it was "difficult to say" if the large slide was a result of climate change, but admitted that if extreme weather like that of last summer continued, such dramatic erosion would happen more often. "That very wet summer last year will have contributed to this," he said. "When you get these conditions, you expect landslides."

Residents are calling for blockades to be put on the cliff face to prevent further erosion along the coast. But Mr Edmonds believes such a move would be disastrous for the protected coastline. "Our concern is that the reason this coast is celebrated is that it is eroding, and if you cover it in rock it will look like Torremolinos," he said.