Leading scientists have written to the Prime Minister demanding he fully commits to a "world-class" network of marine reserves off the Westcountry coast.
The move comes after the Government waivered on proposals to implement controversial marine conservation zones in UK waters – scaling original plans for 127 down to just 31, with only 15 earmarked from 58 off the South West coast.
A two-year project, conducted by Finding Sanctuary in the South West, was part of an £8 million investment nationwide involving conservation groups, experts and marine industries.
It included 65 highly protected areas where all damaging activity such as fishing would be banned.
But the subsequent decision to consult on just 31 areas, with no highly protected or "reference areas" planned, has prompted dismay among wildlife and scientific experts.
In a letter to David Cameron, 86 scientists have warned that even if all 31 marine conservation zones currently under consideration were established, it would not deliver the required protection for ocean wildlife.
"According to your Government-appointed scientific advisory panel, the majority of targets that Government set to achieve an ecologically coherent network of protected areas would only be met if the proposed sites were designated in their entirety," they wrote.
"The network was widely heralded as a foundation for recovery of English seas."
A failure to implement all the proposed areas has thrown the process of creating a coherent network of zones into "confusion and disarray", the scientists told the Prime Minister.
"To restore confidence, we urge you to reaffirm your Government's commitment to establishing a comprehensive, world-class network of marine conservation zones that delivers high levels of protection from damaging activities, especially mobile fishing gears, and to publish a clear timetable for the completion of the network, including reference areas."
The move has been led by Callum Roberts, professor of marine conservation at the University of York, and signatories include Professor Sir John Lawton, who recently led an independent review for the Government on wildlife sites.
One of the scientists, Dr Jean-Luc Solandt, senior biodiversity policy officer for the Marine Conservation Society (MCS), said the UK's seas badly needed protection.
He said: "MCS is seriously worried that the Government has become timid and noncommittal on the issue of marine protection.
"In a cut-down form, there will be no real network and the opportunity to give English marine life the protection it desperately needs will be missed," he warned.