GREENPEACE protesters were yesterday accused of piracy after they removed the antenna of a buoy that was being used in the hunt for new oil reserves in the Atlantic.

The activists took to the sea in inflatable dinghies to sabotage the buoy, as part of a campaign that has also seen protesters spending more than a month living on the inhospitable island of Rockall.

The buoy is being used in seismic tests by the research vessel Pacific Horizon on behalf of a group of oil companies, headed by the Texaco group. The companies are hunting new reserves west of the Shetland Islands.

The ship trails three-milelong "streamers" fitted with seismic guns, which fire shocks into the sea. Echoes of the shocks are picked up by microphones to reveal information about the geological make-up of the seabed.

Without the buoy, there is no way to keep the streamers in place, or to transmit the information that is picked up.

The ship MV Greenpeace has dogged the Pacific Horizon for four weeks in an attempt to disrupt its work.

In recent days, swimmers have been lowered into the chilly Atlantic ahead of the ship, forcing it to change direction and invalidating its tests, which depend on the sheep keeping a straight course.

A Greenpeace spokeswoman said: "This campaign is all about climate change. The world knows that it cannot afford to burn even a quarter of existing fossil fuel reserves without causing climate change, so what is the point of looking for more?

"Someone, somewhere has got to have the sense to stop looking for new reserves and start phasing out fossil fuels."

The incident, which took place yesterday about 190 miles from Rockall, led to the Pacific Horizon giving up its tests for the day, she said.

The spokeswoman added "Our captain, Jon Castle, was accused of piracy, but we have no intention of permanently depriving the owners of the buoy's antenna. We have removed it temporarily."

Mr Castle said: "It is piracy and madness to continue searching for new oil that the climate can never afford to have burnt."

A Texaco spokesman said: "Texaco confirms that illegal action by the master and crew of the MV Greenpeace has resulted in a temporary cessation of the seismic survey on Tranche 36, approximately 60 miles north-west of Lewis.

"It is understood that members of the Greenpeace crew boarded the tail buoy, which was under tow at the end of the seismic string, removed radar and satellite navigation equipment and attempted to overturn the buoy. The equipment was taken to the MV Greenpeace."

He said that Texaco and its seismic contractor, Horizon, would continue to take every step to ensure that no personnel were exposed to risk.

The Texaco spokesman added: "Both companies urge Greenpeace to cease taking action which presents a high level of risk to the safety of their own and the companies' personnel offshore."