The environmental campaigners sailed to the western part of the Channel between the UK and France, loaded with the boulders of Portland limestone, each weighing between 500 and 1,400 kilograms (1,100 and 3,100 pounds).

The giant rocks were dropped on Thursday from its Arctic Sunrise research vessel in an area of the South West Deeps (East) Conservation Zone, which lies some 190 kilometres (120 miles) off Land's End, the most westerly point of mainland England.

"We are placing large limestone boulders on the seabed to create a protective underwater barrier which will put the area off limits to destructive fishing," Anna Diski, UK oceans campaigner, told AFP on board.

The action would make it "impossible for them to drag the heavy fishing gear along the seabed, destroying the habitat and disturbing the carbon", she added.

Artists created a giant ammonite sculpture—inspired by the fossil often found in Portland limestone—out of one of the boulders, which was also placed on the seabed.

The names of the action's celebrity backers and supportive politicians were also inscribed on the rocks.

"Right now, there's an industrial fishing frenzy happening in UK waters, and what's our government doing about it?" asked Greenpeace UK's head of oceans, Will McCallum.