Greenpeace campaigners have chained themselves to a ship which they claim is carrying genetically-modified maize into Britain.
Stalemate: The cargo ship would be unable to move until high tide
Four protesters sailed out to the Etoile as it approached Avonmouth docks near Bristol on Sunday evening.
They climbed up access ladders and are trying to convince the crew to turn round, back to the USA.
The ship is now anchored off Llantwit Major in south Wales, and because of its size must wait until high tide at 2100 BST before it is able to move.
The activists have unfurled banners saying "Stop GM imports" and say they will leave only if the ship returns to the US.
They say using GM maize to feed to dairy cattle is "GM through the backdoor".
One of the protesters on board Ben Ayliffe said: "We're going to stay here for as long as we possibly can. We are well set... we're very committed to making sure this ship turns round with its cargo of GM crops - goes back to the USA."
Greenpeace members made a video of boarding the ship
The 125,000 tonne ship, from Louisiana, was nearly three miles south of Breaksea pilot station, where pilots board ships and steer them into their destination dock, when the activists boarded it.
Greenpeace's Ben Stewart said three members remained on board and about 10 more were in boats around it, preventing it from moving.
"Because we don't think this stuff should be coming into the country we sent three speedboats and got climbers on to the boat," he said.
"We have put a climber on to the pilot ladder and he is locked on to it.
"We are negotiating with the captain and telling him we will only leave when he agrees to go back to America."
Supermarkets have responded through the Retail Federation and say that Greepeace are creating unnecessary food fears through misleading claims.
The Foods Standards Agency, they added, has categorically stated that milk from animals fed on GM crops does not contain any GM material.
The group carried out a similar action in February 2000, when they ambushed a ship carrying GM soya off north Wales.