An Arctic oil rig was forced to stop drilling by environmental activists demanding to know how its owner would respond to an oil leak on the scale of last year's Deepwater Horizon spill.

Edinburgh-based Cairn Energy said drilling was suspended at its Leiv Eiriksson rig off the coast of Greenland after 18 Greenpeace protesters breached a restricted area. The activists launched from the Greenpeace ship Esperanza in five inflatable speedboats from outside a 500m exclusion zone set up by Danish authorities.

Campaigner Ben Ayliffe said: "Cairn Energy is hiding its oil spill response plan, so we're going to the one place where there must be a copy of it. It's obvious why Cairn won't tell the world how it would clean up a BP-style oil spill here in the Arctic, and that's because it can't be done.

"Experts say the freezing temperatures and remote location mean a deep water blow-out in this stunning pristine environment would be an irreversible disaster. If they published the plan, the dangers of investing in such a high-risk venture would be laid bare. We have to draw a line in the ice and stop the Arctic oil rush."

Cairn announced this week that it had begun drilling in two wells in the region. The two wells are approximately 160km and 300km off Nuuk, the capital of Greenland. Each drilling operation is in water deeper than 900m.

Last week, two Greenpeace protesters occupied the company's 53,000-tonne drilling vessel.They hung from the underside of the rig in an Arctic survival pod and had enough supplies to keep them there for 10 days, but were removed by Danish police.

Cairn has asked a court in the Netherlands to legally prevent Greenpeace from disrupting any future deep-sea drilling operations.

In a statement, the company said: "Cairn confirms that members of Greenpeace have boarded the semi-submersible drilling vessel, the Leiv Eiriksson, owned by Ocean Rig. The protesters have breached the safety exclusion zone and entered a restricted area on the rig. In accordance with the strict health and safety practices employed in this drilling programme and in order to ensure safe operating conditions, drilling has been suspended.

"Cairn respects the rights of individuals and organisations to express their views in a safe and peaceful manner, but would be concerned with any action that presents a risk to the safety of people and/or equipment."

The company added: "Wherever it is active, Cairn operates in a safe and prudent manner. The Greenland Bureau of Minerals and Petroleum has established some of the most stringent operating regulations anywhere globally, which mirror those applied in the Norwegian North Sea. Cairn takes its responsibilities such as oil spill contingency and response plans very seriously.

"Cairn, working closely with the Greenland authorities, has developed an extensive emergency response and oil spill response plan. As stipulated by Greenland Authorities, the oil spill response documents are not publicly available."