"Canada will not hesitate to defend its interests, including at the World Trade Organization," state letters sent to European commissioners by Canada's ambassador to the E.U. and its oil minister, released under freedom of information laws.

The move is a significant escalation of the row over the E.U.'s plans, which Canada fears would set a global precedent and derail its ability to exploit its tar sands, which are the biggest fossil fuel reserve in the world after Saudi Arabia. Environmental groups argue that exploitation of the tar sands, also called oil sands, is catastrophic for the global climate, as well as causing serious air and water pollution in Alberta.

Darek Urbaniak, at Friends of the Earth Europe, which obtained the new documents, said: "These letters are further evidence of Canadian government and industry lobbying, which continuously undermines efforts to combat climate change. We find it unacceptable that the Canadian government now openly uses direct threats at the highest political levels to derail crucial E.U. climate legislation."

The unveiling of Canada's threats is the latest in a series of recent embarrassing revelations. On February 12, the occurrence of a secret strategy "retreat" in London in 2011 was discovered. High-level officials discussed the "critical" issue of winning the tar sands argument in the E.U., to "mitigate the impact on the Canadian brand" and to protect the "huge investments from the likes of Shell, BP, Total and Statoil". Representatives of Shell, Total and Statoil attended the meeting alongside the U.K.'s state-owned Royal Bank of Scotland and the Canadian Association of Petroleum Producers.