Work to protect an ancient cliff face from collapsing into the sea started this week.

The Black Rock cliffs behind Asda, at Brighton Marina, date back to the Ice Age and contain the remains of woolly rhinos, wild horses and mammoths.

They have been closed off to the public since 2001, when storms caused parts of the cliff to crumble and fall onto the Undercliff Walk. Part of the walk was fenced off to stop pedestrians being injured and has been off limits since.

During the next 16 weeks a team of professional abseilers from building firm CJ Thorne, based in Uckfield, will be working to sink bolts into the rocks that should hold them in place for the next 50 years.

The works cover a 20 metre tall cliff stretching for 150 metres. It will also include trimming back parts of the cliffs left hanging off after previous rock falls and placing fences at the bottom of the Underpass Walk to collect any landslides.

Deputy Prime Minister John Prescott gave permission for the works to go ahead in order to save the cliffs but ruled out the use of wire meshing to stop further cliff falls.

Because the wire meshing is not being used, the Underpass Walk will remain closed once the work is finished.

Conservation officer Jon Curson, from Government watchdog English Nature said: "This is the best solution we could have hoped for and we are quite pleased with it.

"Although they are putting rock bolts into the cliff face, without them the cliff face would crumble away anyway.

"They are going to take away one of the intrusive walls at the bottom of the cliff face so all in all it is a big improvement."

Chris Webb, in charge of the construction team, said: "The challenge for us is to do everything safely, so we will use professional abseilers to put in the rock bolts."

Councillor Gill Mitchell, chairwoman of Brighton & Hove's environment committee, said: "This is a delicate engineering operation as these cliffs are truly unique.

"But the work must be carried out to stabilise the cliffs for safety reasons.

"The work will protect the seafront road and help to preserve the cliffs for as long as possible."