The UK must not block moves which would effectively ban polluting "tar sands" for use as transport fuels in Europe, green groups have warned ahead of a key EU vote.
Environmentalists said the Government would further damage its "already threadbare" green credentials if it bowed to lobbying from Canada and oil companies against attempts to prevent the unconventional oil being used in the EU.
If tar sands, which produce more emissions than conventional oil due to the energy required to extract them, are fully exploited it will be "game over" for the climate, campaigners warned.
The European Commission has proposed to rule tar sands as more polluting than conventional transport fuels under the Fuel Quality Directive, which commits the bloc to cutting emissions from the production of transport fuel used in Europe by 6% by 2020.
The move could effectively ban the use of oil from tar sands in Europe if the bloc is to meet the directive's target.
Charlie Kronick, senior climate adviser for Greenpeace, said a vote which would stop the use of the polluting fuels in the future would send a strong message to industry and governments in the US and Canada, where tar sands and oil shale are being exploited.
He said tar sands, dug up from the forests of Alberta, Canada, were 25% worse in terms of greenhouse gas emissions than conventional oil, and warned that only a small proportion of the resource had been exploited so far.
But several countries oppose the move by Europe, while the UK and Germany have not formally stated their position. Without backing by the UK and Germany it will not be passed.
Friends of the Earth's energy campaigner Tony Bosworth said: "If the UK backs moves to allow filthy tar sands oil from Canada into Europe it would seriously undermine the coalition's already threadbare environmental credibility.
"Tar sand oil is much dirtier than conventional oil and its extraction has a devastating impact on the Canadian environment - ministers must support moves to block its entry into the EU."