Greenpeace has vowed to continue dropping boulders into a marine reserve to stop harmful fishing after it said the Government had failed to commit to boosting the area’s protection.

The campaign group said it would continue to build the underwater barrier to prevent bottom trawling in Dogger Bank, in the North Sea, despite being warned to stop by the Government’s Marine Management Organisation (MMO).

Some of the boulders now being dropped to help shut off almost 50 square miles of the sea have been signed by celebrities including Stephen Fry, Hugh Fearnley-Whittingstall and Alison Steadman.

The shallow sandbank is protected for its seabed habitat, and is home to crabs, starfish, flatfish and sandeels which are food for seabirds such as puffins, as well as for porpoises and dolphins and fish including cod.

Environmentalists accuse the Government of failing to properly protect the area by not fully restricting damaging fishing activity such as bottom trawling in which heavy weighted nets are dragged over the seabed to catch fish.

Greenpeace has received a letter from the MMO formally requesting it makes no further deposits of the boulders from its vessel Esperanza and that the activity was under investigation.

But the green group said it had received no tangible commitments from Government to protect the Dogger Bank and so had no choice but to continue.

Chris Thorne, Greenpeace campaigner, also warned that Prime Minister Boris Johnson’s promises to protect 30% of the UK’s land and seas were “meaningless” without concrete plans to deliver on the targets.

“If our Government is not willing to commit to proper protection for the Dogger Bank and the rest of the UK’s Marine Protected Areas, we are forced to continue doing all that we can to prevent bottom trawling from destroying this vital marine habitat.

“We can’t let bottom trawlers, which often operate illegally with their positioning systems off, continue to rip up the protected seabed while our Government does nothing.

“We will not sit idly by while our oceans are destroyed.”

Greenpeace said the inert boulders, which they were spacing at precise intervals inside the Dogger Bank protected area, would not have a significant impact on the seabed but would stop destructive bottom trawling.

Any bottom trawlers trying to fish over the boulders will get their gear snagged and ruined on the rocks, though passing marine traffic will not be affected.