The Altair, flying the Dutch flag, entered the harbour with four inflatable vessels to try to keep the destroyer Dupleix in port. Activists painted "Stop nuclear tests" on the side of the Dupleix and chained themselves to the moorings of the ship, which was in Brindisi for Nato exercises.
Giuseppe Onufrio, a Greenpeace activist on board the Altair, said: "French commandos ... boarded our boat, broke the glass of the windows in the bridge and threw in six tear-gas grenades. The French have committed an act of war in Italian waters."
Witnesses said the crew of the Altair seemed to lose control of the ship for about 10 minutes after the commandos boarded. The commandos had axes and forced the Greenpeace crew to abandon ship.
Sailors on the Dupleix used water cannon to swamp the inflatables and flood the Altair's engine-room. The French then started the engines of the abandoned ship, sending it crashing against the dockside. The Altair's rudder was damaged, Greenpeace said. Some Italian MPs asked for an investigation of the French retaliation.
The Altair was commanded by an Englishman, Dave Enever, who was arrested in July when in command of Rainbow Warrior II when she was boarded by French commandos near the nuclear test site at Mururoa.
Captain Enever, from Frinton, Essex, said: "I couldn't believe that in an Italian port another member of the EU could do something like that. They threw tear-gas grenades, smashed windows and poured tons of water down the funnel and into the engine-room. There's so much water between decks you get a free shower every time you move." Capt Enever said the commandos were "doing their hard-man act".
"While we were off the bridge they put the engines into reverse and the ship almost demolished an Italian Customs boat before she hit the dock. We are holed above the waterline and we won't be sailing anywhere until repairs are carried out."
Two other Britons on board the Altair yesterday were named as Geoff Sheldon and Elizabeth Blake. Greenpeace organised protests against France after it decided to resume nuclear testing in the South Pacific.