Five years without fishing around Lundy Island off the coast of Devon have brought a significant revival in sea life, scientists report.
Lobsters are seven times more abundant within the protected zone than outside.
The eastern coast of Lundy is the UK's only "no-take" zone, where fishing is completely prohibited.
Conservation groups say UK seas need more of them, but the government's recent Marine Bill promises much vaguer "marine conservation zones".
It is not clear what levels of protection these areas would have. The site wasn't only set up to protect lobsters - it's to protect the whole environment
Chris Davis, Natural England
The Lundy zone was set up five years ago by Natural England and the Devon Sea Fisheries Committee, which administers fishing along the county's coasts, in partnership with local fishermen.
Natural England scientists believe the zone should help Devon's lobster-potters by providing a refuge where young lobsters can grow to maturity, then migrate into areas where commercial fishing is permitted.
On the up
"The main result we have seen is an increase in the number of large lobsters in the no-take zone compared to areas where fishing is on-going," said Miles Hoskin, the marine biologist engaged by Natural England to lead the research.
There are more lobsters caught inside the zone, and they are larger
Recent surveys have found that lobsters above the minimum landing size are between six and seven times as abundant within the zone as outside.
"In recent years we've also found an increase in the number of small lobsters within the zone and adjacent to the zone," Dr Hoskin told BBC News.
"In the next year or two they're all going to be lobsters that fishermen can catch."
The team surveys five sites - one in the no-take zone, two commercially fished sites around Lundy, and two comparison sites further afield, one on the north coast of Devon and one in South Wales.
Surveying consists of laying and then retrieving strings of commercial lobster pots, and counting and sexing the animals inside.
The approximate doubling in numbers of young lobsters has not been seen at the two distant sites, suggesting that it is a consequence of the no-take zone.
Scientists are now putting tags on the lobsters they catch. Fishermen are being encouraged to report catches of tagged animals, in order to show how far they are migrating out of the no-take zones.
Fishermen are generally cautious about no-take zones, which is one reason why the government plumped for the much more adaptable "marine conservation zone" concept in the draft Marine Bill.
"It's difficult to to say whether it's helped us - we didn't used to fish in there much anyway, except close to shore, but it was always good for lobsters," said John Barbeary, whose lobster and whelk boat works out of Ilfracombe.
"When we were asked about it we were all for it... (but) we couldn't afford to have the zone made any bigger because it would completely ruin our business, and I think you'd find that with a lot of fishermen around the country - it would make it totally uneconomic."
But Sarah Clark from the Devon Sea Fisheries Committee said she believed the zone was good for the industry.
"Having a larger brood stock especially of females within the no-take zone will obviously produce more juveniles," she said.
"We're tagging them to see if they're moving out - if they are, they'll be moving out of the no-take zone into the area that's being fished, and and that can only help with the fishery, and help fishermen too."
Natural England's root reason for wanting the zone closed was not to help fishermen, but to return a tiny fraction - 0.002% - of the UK's seas to the state they were in before the era of modern fishing.
"The site wasn't only set up to protect lobsters - it's to protect the whole environment," said Chris Davis, the agency's senior specialist in marine policy.
A crayfish is a surprise catch - "the first in five years"
"It's about protecting the fish and the sponges and the coral that's here as well, and it's doing a good job, though it's a bit difficult to say on some of the species because they don't reach maturity for 30 or 40 years."
A by-product of nature protection may be an increase in the tourist trade. A full analysis has yet to be done, but anecdotally the numbers of divers visiting Lundy has risen.
However, the views of fishermen are likely to be highly influential when it comes to deciding how many of the new marine conservation zones - which are several years away from being proposed - acquire full protection.
So will the views of the burgeoning renewables industry, given the potential of UK seas for generating electricity through tidal and wave technologies as well as offshore wind turbines.