All but one in the group were part of a "substantial team" who used four boats to block the ship, the MV Etoile, off the Welsh coast on Sunday as they forced their way on board, Greenpeace spokesman Ben Stewart said.

Some of the activists hung from ropes at the rear of the ship, while others climbed the front mast with an anti-GM banner and rode the boat in as it docked Tuesday in Bristol, in the west of England, where they were arrested.

A 13th person was arrested by police Monday in a raid on a hotel outside the Welsh capital of Cardiff, Stewart said.

Greenpeace claims that the 125,000-tonne MV Etoile, one of the world's largest freighters and registered in Panama, was loaded with US-grown GM maize intended to feed British dairy cows.

In line with new European Union rules, food for human consumption and animal feed must be labelled if they contain at least 0.9 percent GM ingredients.

But milk, meat and eggs of animals reared on bio-engineered products -- widely used in feed across Europe -- do not have to be.

"The reason why these imports are coming in is because supermarkets (in Britain) are selling milk from cows that are fed on GM crops," Ben Ayliffe, one of the activists arrested, told AFP by telephone from his boat on Monday.

"The United Kingdom can never be GM-free until these imports stop," he said.

The Greenpeace spokesman said it was "inconceivable" that the freighter was not loaded with GM maize, since it came from the southern US state of Louisiana where crops were not "segregated between GM and non-GM".

"Shipments of non-GM maize do not cross the Atlantic between America abnd Europe," he said.

The environmental pressure group has staged similar forced blockades of GM-carrying vessels in the past, temporarily blocking two GM shipments in Greece in May and shutting down two Italian ports on the Adriatic Sea in May to keep out GM soy.

"Greenpeace will continue to use non-violent action... to stop environmental threats," Stewart said, citing what he called rampant environmental damage from overuse of pesticides with GM crops.

TERRA.WIRE