Major international oil companies are buying off governments, according to the world's most prominent climate scientist, Prof James Hansen. During a visit to London, he accused the Canadian government of acting as the industry's tar sands salesman and "holding a club" over the UK and European nations to accept its "dirty" oil.

"Oil from tar sands makes sense only for a small number of people who are making a lot of money from that product," he said in an interview with the Guardian. "It doesn't make sense for the rest of the people on the planet. We are getting close to the dangerous level of carbon in the atmosphere and if we add on to that unconventional fossil fuels, which have a tremendous amount of carbon, then the climate problem becomes unsolvable."

Hansen met ministers in the UK government, which the Guardian previously revealed has secretly supported Canada's position at the highest level.

Canada's natural resources minister, Joe Oliver, has also visited London to campaign against EU proposals to penalise oil from Alberta's tar sands as highly polluting. "Canada can offer energy security and economic stability to the world," he said. Oliver also publicly threatened a trade war via the World Trade Organisation if the EU action went ahead: "Canada will not hesitate to defend its interests."

The lobbying for and against tar sands has intensified on both sides of the Atlantic as the EU moves forward on its proposals, which Canada fears could set a global precedent, and Barack Barack Obama considers approving the Keystone XL pipeline to transport tar sands oil from Canada to the US gulf coast refineries and ports. Canada's prime minister, Stephen Harper, was met by protesters when he visited New York last week to tell audiences that KXL "absolutely needs to go ahead".

Canada's tar sands are the third biggest oil reserve in the world, but separating the oil from the rock is energy intensive and causes three to four times more carbon emissions per barrel than conventional oil. Hansen argues that it would be "game over" for the climate if tar sands were fully exploited, given that existing conventional oil and gas is certain to be burned.

"To leave our children with a manageable situation, we need to leave the unconventional fuel in the ground," he said. Canada's ministers were "acting as salesmen for those people who will gain from the profits of that industry," he said. "But I don't think they are looking after the rights and wellbeing of the population as a whole.

"The thing we are facing overall is that the fossil fuel industry has so much money that they are buying off governments," Hansen said. "Our democracies are seriously handicapped by the money that is driving decisions in Washington and other capitals."

The EU aims to penalise oil sources with higher carbon footprints, as part of a drive to reduce the carbon emissions from transport called the fuel quality directive (FDQ). But Canada, supported by the UK, is fiercely opposed: "We are not saying they should not move to reduce emissions," said Oliver. "But the proposed implementation of the FQD is discriminatory to oil sands and not based on scientific facts." However, Europe's commissioner for climate action, Connie Hedegaard, said the FQD was "nothing more, nothing less" than accurate labelling and putting a fair price on pollution.

Hansen, who informed the US Congress of the danger of global warming in 1988, has caused controversy before by saying the "CEOs of fossil fuel companies should be tried for high crimes against humanity" and calling coal-fired power plants "factories of death". In April, he stepped down from his Nasa position after 46 years, in order to spend more time communicating the risks of climate change and to work on legal challenges to governments.

Hansen has started a science programme at Columbia University, the first task of which is to produce a report to support suits filed again the US federal government and several state governments. It is being pursued by the Our Children's Trust charity and is based on a trust principle recognised in US law.

"We maintain that the atmosphere and climate are held in trust by the present generations for the future generations and we do not have the right to destroy that asset," Hansen said. "Therefore the courts should require the government to give a plan as to how they are going to ensure that we still have that asset to pass on to the next generation."