Scientists and ministers were at loggerheads last night as they blamed one another for letting plans to protect Britain's marine wildlife and habitats descend into "confusion and disarray".

An open letter signed by 86 scientists and academics sent to David Cameron accuses him of falling "far short" of delivering a pledge to create a network of Marine Conservation Zones (MCZs). The group claims plans to establish a network of 127 protected areas have been watered down to such a point that the zones "will not deliver" the level of protection needed by declining wildlife and their habitats.

The zones were identified as the areas of the sea that most needed protection from fishing and other commercial interests in order to provide safe havens for hard-pressed wildlife. Signatories to the letter include 31 professors, four Fellows of the Royal Society, and five of the scientists who served on the Government's own advisory panel on the subject. However, ministers immediately hit back, claiming that the scientists have failed to come up with the evidence required to support the need to create the zones, leaving the Government in the position that it could only put 31 sites out for consultation instead of 127.

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Richard Benyon, the environment minister in charge of the MCZ scheme, fears a raft of legal challenges from fishermen and other vested interests in the UK and Europe if MCZs are created without heavyweight science to justify them. He was particularly critical of five signatories of the letter who also sat on the Government's Science Advisory Panel on MCZs whom he said had concluded there was too little scientific evidence to designate 127 sites - a claim disputed by the scientists. "Some of the scientists who have signed this letter actually sat on the panel that told us there was not enough evidence to designate all 127 sites," he said.

He added of the 86 signatories: "Rather than jumping on the bandwagon and lobbying Government, I'd ask these scientists to focus their attention on gathering more evidence so we can designate more sites in future."

His attack further incensed marine scientists. Professor Callum Roberts of the University of York said: "The Government is trying to palm off the blame for the decision. They need to have the balls to stand behind it rather than saying the science was lacking."

Professor Roberts said the letter, which he organised, was an expression of how frustrated, angry and let down marine scientists feel. "It was clear when I was contacting scientists about this letter that there was a very great deal of anger and upset," he said.

"We had within our grasp a world class network of protected areas which would have gone a very long way towards the recovery of England's marine life and productivity. Now it's a complete mess."

The row escalated little more than a week after the House of Commons Science and Technology Committee concluded it appeared the Department for Environment, Food and Rural Affairs "moved the goal posts" on what was expected of the scientific evidence on MCZs. In the original process of identifying sites the "best available evidence" was requested but afterwards more robust and comprehensive evidence was demanded.

In the open letter, scientists urged the Prime Minister to "reaffirm your government's commitment" to establishing a "world-class network" of MCZs protected from damaging activities including fishing.

Marine wildlife on the critical list

Common skate

It was common in the 19th century but has declined by 99 per cent. It is classified as critically endangered by IUCN. It has almost gone from English waters. "It's doomed without this network," said Professor Callum Roberts.

Fan mussels

Used to be widespread in UK waters, but the advent of bottom trawling and dredging have caused them to disappear in most places. Some pockets survive near wrecks. They have declined 99 per cent since the 19th century.

Maerl

A coralline algae that builds up like corals build up into reefs. Maerl beds are valuable habitats for a range of other species. "These are species that have been pushed towards the edge," said Professor Roberts.

Atlantic halibut

The fish have declined by 95-99 per cent since the 19th century. They are classified as endangered by the IUCN.

Wolffish

They have declined by 95 per cent since the 19th century in English waters.