Workers on BP's Foinaven production vessel, 100 miles west of Shetland, spotted an "oil slick" shortly after the MV Greenpeace left the area.
A spokesman for BP, which provides 17 per cent of Britain's oil and 13 per cent of its gas, said: "The Greenpeace vessel sailed past in open seas outside of the exclusion zone around our site. I don't know exactly what they were doing there, just watching us, I suppose. But when they sailed off there was a sheen on the water. We have a statutory duty to report such things to the Department of Trade and Industry."
BP staff took photographs and samples of the tainted water for investigation. Although the leak was small, BP stood by the decision to report it, the spokesman said.
A spokesman for Greenpeace, which has been campaigning against BP's North Atlantic production sites, said the spill was probably caused by rain washing oil off the decks.
He said the authorities considered the incident insignificant and took no action. "The report to the Marine Pollution Control Unit indicated that less than a litre was seen."Reuse content