A walkway under crumbling cliffs is likely to be closed for good because a council cannot find a way to make it safe.

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

Conservationists had feared a £500,000 programme by Brighton and Hove City Council to preserve the cliffs at Black Rock, Brighton, would cause severe damage.

Deputy Prime Minister John Prescott said the work could go ahead but insisted on strict conditions, including forcing the council to drop plans to mask the cliffs with mesh.

The verdict followed a public inquiry into the scheme, drawn up to prevent repeats of cliff falls which forced workers and shoppers to flee the Asda store in April 2001.

The council yesterday announced it was applying for permanent closure of the Undercliff Walk, although the full outcome of the public inquiry is not yet legally binding. A spokeswoman said the authority had been prepared to invest in a scheme to make the cliffs safe and reopen the walk.

However, English Nature said the cliffs were of such archaeological importance they should not be covered.

Councillors said the decision was being made reluctantly.

Environment committee chairwoman Gill Mitchell said: "We have fought long and hard to try to resolve this issue.

"We recognise the historic and geological importance of the cliffs and our proposals were to carry out trimming and stabilisation works while covering the site as much as possible.

"Unfortunately, the view of the Secretary of State for Environment was that we could only carry out part of the work, which means we cannot guarantee safety for walkers."

Rottingdean councillor Lynda Hyde said: "It is regrettable the council was not allowed to carry out remedial works to keep the walk open.

"It seems the Government take more notice of English Nature than city councils."

The council has so far had to foot the bill for clearing up the rubble from the 2001 cliff falls and installing temporary fencing.

Closure of 200 metres of the Undercliff Walk means people will have to walk round Asda and rejoin the path near the Master Mariner pub.

Official notices advertising the council's intentions to close the walkway will be published this week.

Permanent closure is being sought for January 2005.

The cliffs are one of the most protected in the country and have triple Special Site of Scientific Interest protection.

Bob Edgar, the English Nature conservation officer for Sussex and Surrey said: "I cannot stress how archaeologically important the cliffs are.

"It is vital archaeologists have access to them, which would be denied if they were covered."