Three stretches of water off the English coast are to get the strictest possible environmental protections, as part of new measures to restore the health of the sea.
Fishing will be banned along with all activities that damage the sea bed, such as mining and laying cables.
Proposals to bring in Highly Protected Marine Areas (HPMAs) at two other sites have been dropped.
Critics say the plans lack ambition and progress is far too slow.
Prof Callum Roberts, of the University of Exeter, said the protected areas would make a big difference to the sea-life within them but covered only 0.5% of English seas.
"At this rate of progress it'll take 260 years to get to the level of protection that science says is needed - which is 30% of the seas highly protected," he said.
The Wildlife Trusts said they would be calling on the government to designate more fully protected marine areas as soon as possible.
"I'm totally dismayed that the government has only decided to designate three of the five proposed HPMAs," said head of marine conservation, Dr Lissa Batey,
"They're a good first step but we really need to do more if we're to turn the tide on nature's recovery and bring wildlife back."
Richard Hill, of the Marine Conservation Society, echoed her words: "It's a step forward, but a baby step," he said. "It's a shame the government isn't being a bit more ambitious in protecting our seas."
The areas to be given full protection are:
Proposals at Northumberland's Holy Island were dropped after fears were voiced that a ban on fishing could devastate the local economy.
A similar plan for Trevelyan Inner Silver Pit South, 16 miles (26km) off the coast of Lincolnshire, was also ditched. The area is a fishing ground for cod, plaice, herring and sole.
The new scheme gives three areas off England full protection against damage from fishing, mining and development - part of a target to protect 30% of marine waters by 2030.
A spokesman for the Department of Environment, Food and Rural Affairs said: "Highly Protected Marine Areas will offer the highest levels of protection in our seas, helping a wide range of marine habitats and species to fully recover."
In Scotland, ministers have committed to designating at least 10% of Scotland's seas as HPMAs by 2026.
Seas are under pressure following decades of over-exploitation, pollution and climate change. While 38% of seas around the British Isles are designated marine reserves, many are still subject to damaging fishing practices such as bottom-trawling, where fishing gear is dragged along the sea bed.
pursue Helen on Twitter @hbriggs.