British motorists are unwitting users of diesel and petrol derived from the tar sands of Alberta, Canada, where carbon-heavy production methods make extraction particularly damaging to the environment, Greenpeace claims.

The environmental group is calling for action by the European commission to strengthen fuel-quality directive regulations to restrict the import of petroleum products made in a carbon-intensive way.

The move comes as the tar sands producers appear to be trying to use the BP oil rig disaster in the Gulf of Mexico as a public relations tool to promote their industry over deepwater drilling.

In a report out on Tuesday Greenpeace says that it has spent time tracking tar sands crude over 12 months and believes that considerable quantities are now being exported to Europe, via refineries in the southern states of the US.

While City investors have begun to question the role of companies such as BP and Shell in the tar sands business, British environmentalists – and consumers – have tended to believe that Alberta crude is used only in North America.

But the Greenpeace report, entitled Tar Sands in Your Tank: Exposing Europe's role in Canada's dirty oil trade, comes to different conclusions.

"The reach of tar sands crude is wider than previously thought. In fact, petroleum products derived partly from tar sands crude oil have been regularly entering the European Union's petroleum supply chain," it concludes.

The environmentalists believe this practice will become more widespread. One company claimed to be at the centre of the trade, the US refiner Valero Energy, plans to increase supplies at its Port Arthur refinery significantly via a controversial new pipeline from Canada to the US Gulf coast, where the BP-leased Deepwater Horizon rig sank, causing a huge oil leak.

Greenpeace also believes that BP has refined at least one consignment of tar sands crude at the Texas City plant on the same coast, which is a regular location for exporting diesel to Europe.

The rival oil producer ExxonMobil is also handling Canadian tar sands at its Baytown refinery near Houston and has exported at least one diesel shipment to Europe over the period studied, according to Greenpeace.

The green group admits that it cannot ultimately prove that any particular consignment derived from Canada ended up in Britain or Europe, given that refined diesel or petrol is of a uniform quality, but it says that the weight of evidence firmly points in this direction.

Exxon said that it could not comment on tar sands refining or exporting.

"The crude oils we process at our refineries come from a variety of sources around the world," said a spokesman. "However, what types of crude are processed at each refinery, how much, and when are all details that we do not discuss publicly as a matter of practice."

Valero confirmed that it was part of a project to expand the Keystone pipeline from western Canada down to the US Gulf Coast. "Once the Keystone pipeline expansion is complete in 2012 or 2013, Valero expects to be one of the largest recipients of heavy crude oil from the project," a company spokesman said. He added that much of that oil would be refined at Port Arthur, which is geared up to process heavy crude.

Valero also confirmed that its refineries were exporting diesel to Europe, but said that: "Exports of gasoline [petrol] from the US to Europe are rare, since Europe usually has an oversupply of gasoline."

BP, which is investing heavily in tar sand production and upgrading refineries near the Great Lakes specifically to refine this crude, had no comment to make on any existing exports.

Glen Schmidt, chief executive of Laricina Energy, part of the industry lobby group In Situ Oil Sands Alliance, told the Edmonton Journal in Canada that while there were sometimes failures with conventional oil and tar sands projects, "the damage would be much smaller and more modest than with offshore spills".

Similarly an editorial in the Calgary Herald said: "Anyone assessing the risks associated with drilling offshore versus the oil sands is going to be looking at things much differently today than he would have last week. All of a sudden it's a choice between risks that are quantifiable versus those that are unknown."