They were the epitome of seaside cuisine - as affordable and accessible as warm beer, fish and chips or a stick of Brighton rock.

But stocks of jellied eels are now in such short supply they could soon become as exclusive and expensive as caviar.

Once available at every beach in Sussex, the snack is being threatened by a huge decline in the eel population.

Sean Ashworth, of the Sussex Environment Agency, said: "From the eel's point of view it is quite a crisis.

"The eel population in Europe has been reduced to one per cent of its Sixties levels. It has been obliterated."

Experts say the eel, traditionally boiled and congealed in its own juices for the consumption of sunbathers and East Enders, has been hit hard by interruptions in its food supply and climate change.

Eels feed on white clawed crayfish native to Sussex waters. But these prawn-like creatures have diminished, thanks to an invasion of North American signal crayfish competing for the same foods.

Eels migrate all the way to Mexico to spawn, a journey that probably takes about a year each way.

In recent years fewer have been making it back, possibly because global warming is slowing the cold currents in the Gulf Stream they rely on.

Eels have also been harmed by a parasite caught from imported Japanese eels which have escaped from Billingsgate market into the Thames.

They are an important part of Sussex's eco-system. There are no otters left in the county, partly because waterways were so polluted in the Eighties but also because otters feed on eels.

If the eel population continues to fall, it is not known what further consequences there will be.

Dr Ashworth said: "An aquatic ecosystem is a complex web of species and interactions. If you impact or remove one of those species then it is very difficult, if not impossible, to gauge the final outcome."

Jack Mills, a fish smoker who runs the Smokehouse shop at King's Road Arches, Brighton, said he knew rod-and-line fishermen had been finding it hard to catch eels.

Jellied eels are still popular, especially with tourists from London, but many sold in Brighton are caught in the Thames and prices have gone up.

Mr Mills, 72, from Lindfield, said: "It is £2.50 for a small portion. They were very cheap. It was six pence a tub."

Mike Watts, of Watts & Son fish shop in St James's Street in Brighton, buys his eels from an importer who sources them in The Netherlands, Ireland and New Zealand.

He said: "They are still popular but probably more with the upper age group.

"I used to keep live eels but there was an abundance of eels then."