A surge in US exports of crude oil to Europe this year has been heralded as the beginning of an alarming rise in highly polluting tar sands imports by environmentalists.

Strict laws prevent the US from exporting its own crude to Europe but it has been re-exporting Canadian crude to EU countries such as Italy, Spain and the Netherlands, since April 2014.

Figures released by the US Energy Information Administration (EIA) last week show that in the first nine months of 2015, shipments to Europe increased by 73%, reaching 3.56m barrels of crude, compared to 2.06m in the year before until November.

While the EIA provided no breakdown of the various types of crude, analysts told the Guardian that they almost certainly contained a high degree of tar sands oil.

“Almost any crude that is being re-exported from the US would most likely be sourced from tar sands,” said Shelley Kath, an independent analyst for the National Resource Defence Council in the US. “It is a very solid assumption because that is the very large production area and it is precisely the type of crude that has trouble finding [other] markets.”

Canada’s tar sands are the world’s second largest carbon reserve after Saudi Arabia and the ex-Nasa scientist James Hansen once said that their exploitation would mean “game over” for the climate.

Some analysts cautioned that it is difficult to make judgments based on the US data, which does not distinguish between different sources or types of crude oil.

“I think there have been some re-exports of Canadian crude but not large amounts,” said Sean Cronin from Argus global markets.

Last year, the NRDC estimated that tar sands imports to Europe could rise from 4,000 barrels per day (bpd) to 700,000 bpd without EU action.

The spike in crude imports now coincides with a decision by the European commission in October 2014 to abandon efforts to legislate against tar sands according to their greenhouse gas intensity, following intense lobbying pressure.

“This is the opening of the gates after the EU’s failure to tackle tar sands in real barrels of oil,” said Colin Roche, a spokesman for Friends of the Earth Europe. “We just hope that they will not be flood gates.”

A Canadian Association of Petroleum Producers (CAPP) report in June noted “growing interest” from Europe in crude manufactured from the country’s tar sands.

The paper said that the EU’s fuel quality directive would have “discriminated against Canadian oil sands crude” but that this was scrapped after objections from the Canadian government and industry.

As a result, “exports of Canadian crude oil to Europe have begun to occur and expanded transportation infrastructure in Canada with proposed pipelines to the coast will lead to increased exports in the future,” the report said.

More than two-thirds of Europe’s oil refineries have already been upgraded to process tar sands, according to a report by the petroleum refining consultancy, Mathpro.

The study found that 71 of Europe’s 95 refineries can now refine heavy or reprocessed crude, in an indication that Europe’s industry is anticipating an imminent switch in fuel imports.

One recent National Resources Defence Council analysis predicted that between 5.3% and 6.7 % of Europe’s crude oil and transport fuels would come from North America by 2020. The figure was 30 times higher than the EU’s estimate, when it abandoned attempts to make oil companies account for their carbon emissions in their final blends.

Tar sands are natural bitumen deposits which produce 23% more greenhouse gas emissions than conventional oils, due to their energy and water-intensive production process.

This involves steam injection and strip mining, which can leave environmental scars ranging from clear-cut forests to toxic tailing ponds and destroyed farmland.