The British oil company leading a controversial drive to discover new oil and gas reserves in the pristine waters off Greenland has come under fire from environmentalists and again disappointed investors after failing to find anything with its sixth well.

Shares in Cairn Energy lost 6.5%, closing at 276p, after it told the London stock exchange that its Delta-1 hole in the West Disko area had been abandoned "without encountering hydrocarbon shows". Six months ago the shares were changing hands at 462p.

Greenpeace also claimed the Edinburgh-based company had irresponsibly pumped more "red-listed" hazardous chemicals into the ocean than the entire oil operations of Norway and Denmark combined.

The campaigners claim more than 225 tonnes of toxic materials – described by the Ospar convention as giving "reason for suspicion because of several polluting effects on the environment" – had been leaked into the Greenlandic Sea this year by Cairn.

"The only thing they've achieved is unnecessarily polluting an important wildlife area with thousands of tonnes of hazardous chemicals," said Greenpeace energy campaigner Vicky Wyatt. "They've flushed away millions that could have gone into profitable clean energy businesses, and no doubt their investors will now be thinking twice about wasting any more money on their dangerous venture," she added.

Greenpeace has been running a high-profile campaign to protect the Arctic, and earlier this year forced the release of Cairn's secret oil spill response plan, which experts derided as inadequate.

Cairn said all chemicals used were approved by the Greenland government, while all drilling operations were being undertaken to the "most stringent" standards in line with those of Norway and Britain.

The Arctic is home to a host of unique and endangered species such as polar bears and walruses, but is also believed to hold as much as a quarter of the world's remaining oil and gas reserves.

Drilling off Greenland has been at the forefront of the new drilling initiatives but oil companies are also working off the far north of Russia, and Shell wants to drill off Alaska.

Increasing interest in the region is reflected by the fact that America sent a top-level delegation for the first time to an Arctic Council meeting earlier this year, and ExxonMobil has just signed a groundbreaking deal with Russian-state owned Rosneft to explore for oil in the Kara Sea. BP earlier tied up a similar arrangement but found it could not proceed because of opposition from its existing Russian partners inside TNK-BP.