Norwegian authorities had given the rig, Eirik Raude, permission to continue drilling after preventative measure had been put in place.

It was the third spillage involving the rig since February.

"The measures implemented before drilling resumed have been adopted in close consultation with the authorities," Statoil official Tim Dodson said.

"By taking these steps, we're minimising the possibility of future undesirable incidents."

In December 2003 the Norwegian government, faced with reduced oil stocks in the North Sea, re-allowed test drilling after a hiatus of some years, but made it dependent on pollution free drilling.