The giant offshore windfarms backed by Andy Burnham this week may face unexpected stumbling blocks after Greenpeace activists unwittingly dumped about 15 tonnes of boulders on to the site as part of a campaign against seabed trawling in the North Sea.

Greenpeace created the “boulder barrier” to safeguard harbour porpoises off the Norfolk coast by dropping giant stone blocks, each weighing up to 2 tonnes, on to the sea floor to prevent a method of fishing known as bottom trawling.