The protest affects a portion of the South West Deeps (East) Marine Protected Area, 118 miles (190km) off the coast in the Western English Channel.
Campaigners and crew on board the Greenpeace ship Arctic Sunrise transported and deposited the 18 limestone boulders in a bid to prevent bottom-trawling.
The Marine Management Organisation (MMO) has said that the action is potentially illegal and has launched an investigation.
The environmental group’s action took place days after UK leaders failed to help secure a Global Ocean Treaty at Intergovernmental Conference 5 in New York.
Greenpeace says this threatens the Government’s aim to achieve at least 30% ocean protection by 2030.
In the last 18 months, the South West Deeps experienced almost 19,000 hours of industrial fishing, 3,370 hours of which was bottom-trawling, according to Global Fishing Watch data. The majority of industrial fishing vessels in the area were from France (53%) followed by Spain (30%) and the UK (9%).
The names of high-profile figures who support the action, including Simon Pegg, Stephen Fry, Conservative MPs Henry Smith and Sir Peter Bottomley, were stencilled onto the boulders before they were dropped.
Will McCallum, Head of Oceans at Greenpeace UK, said: “Greenpeace UK has created this underwater boulder barrier as a last resort to protect the oceans. We’d much rather the Government just did their job.
“Our new Prime Minister must protect local fishing communities and immediately ban industrial fishing in Marine Protected Areas by tweaking commercial fishing licences.”
A spokesperson from the MMO said: “We are very disappointed that Greenpeace has chosen to take this action to drop boulders to form a barrier as we don’t believe it is justified or will help protect our marine environment.
“It is also a requirement under the Marine and Coastal Access Act that any deposit of construction below high water is required to have a Marine Licence. Greenpeace do not hold, nor have they applied for one, and as such their activity is potentially illegal.
Billions of people rely on healthy oceans, and world leaders failed all of them.
The UN failed to agree to a Global Ocean Treaty.
We won’t stop until we have protection for our blue planet. pic.twitter.com/TcV0sg1GG6
— Greenpeace (@Greenpeace) September 1, 2022
“As unlicensed activity has now taken place, MMO has launched a formal investigation in accordance with its regulatory function around marine licensing enforcement and is gathering evidence before considering its next response.”
Barry Deas, CEO of the National Federation of Fishermen’s Organisations, said that “boulders of that size represent a threat to the crews and the vessels that fish the area”, adding that they could be “a potential threat to life”.