More than 50,000 farmed fish have been released into a reservoir in an act of vandalism thought to be linked to animal activists.

Managers at Bewl Water near Wadhurst believe thousands of rainbow trout worth £100,000 will die as a result of the sabotage.

And anglers fear it could mark an escalation in animal liberation campaigns against caged animals.

Fish farm workers awoke to find nets holding young fish being reared for angling had been unhooked on Thursday morning.

It is believed the saboteurs must have rowed out to the pens in the middle of the lake during the night to release the prized trout.

Estate manager Howard Mackenzie said: "A lot of the fish won't survive and it seems to be rather more planned than an act of random vandalism.

"We don't know if it was animal activists or not but it was without a doubt deliberate."

Bewl Water is the largest inland lake in the South-East and the fish were being reared for the start of the angling season in March.

Only about 20 per cent were ready to be released. Managers believe the rest will probably be eaten by larger fish or birds.

No one has yet claimed responsibility for the act. Angling organisations said they fear an animal liberation group had struck at a freshwater fish farm for the first time.

Anglers have long believed they would become the focus of such attacks after the eventual ban on hunting with dogs was passed in Parliament.

Police have said there was no indication whether it was the work of vandals or animal rights groups.

A spokeswoman for Southern Water said the fish, some only a few ounces, would have been gradually released throughout the season, according to size.

She added: "We don't know who did it and police are investigating but whoever they were they were very determined. It was freezing cold that night.

"It could have been kids but I can't believe anyone would have bothered to do this if they were bored."

Bewl Water is currently only 36 per cent full, following the region's worst drought in nearly 100 years.

However, a permit was granted on Friday to allow more water to be pumped in from the River Medway, refilling the reservoir.

Environmentalists voiced fears that the move could endanger wildlife.

Brenda Pollock, South- East co-ordinator for Friends of the Earth, said: "Solving a problem by extracting more and more water can't be good for the environment in the long term. People need to start looking at how to adapt to changing weather patterns."

Meyrick Gough, Southern Water's water planning and strategy manager, said: "Only three of the last 14 months have seen average rainfall. Bewl is a key resource. It is vital that we get it as full as possible and we envisage the level should be raised to about 90 per cent by the end of March."