Attempts made by the previous Labour government in the U.K. to get millions of people to switch from petrol cars to diesel vehicles in order to "save" the planet was a mistake, Barry Gardiner, shadow Environment Minister admitted this weekend.

"Hands up - there's absolutely no question that the decision we took was the wrong decision. But at that time we didn't have the evidence that subsequently we did have," Gardiner said during an episode of Channel Four's Dispatches, called "The Great Car Con."

At least ten million Britons are driving diesel cars, a trend which was encouraged by tax breaks by Gordon Brown back when he was Chancellor of the Exchequer. Brown said back in 1998 that diesel vehicles will attract less vehicle tax than petrol options due to their "better CO2 performance."

Brown reduced vehicle tax for all vehicles with low CO2 emissions in 2001, giving company car buyers, responsible for half of new purchases, a reason to make the switch.

"We also (expected) cleaner diesel engines, which we thought meant that any potential problem was a lower-grade problem than the problem we were trying to solve of CO2," Gardiner added.

In 2014, more than half of all new vehicles sold were diesels, according to The Telegraph. While diesel cars may have lower CO2 emissions than petrol cars, they emit a higher amount of deadly pollutants, like nitrogen dioxide and sooty particle matter, both of which have contributed to high levels of air pollution resulting in deaths of 29,000 people every year.

The policy was a response to the 1997 Kyoto treaty, which was created to cut greenhouse gases.

"It was right to move away from vehicles that push out CO2, but the impact is a massive public health problem," said Gardiner. "The real tragedy is after we set up the committee on the medical effects of air pollution and it reported back in 2010 we've had five years that this government has done nothing about it."

Government ministers were warned more than 20 years ago of the risks, according to Professor Roy Harrison, professor of environmental health at the University of Birmingham.

"I chaired an advisory committee in 1993 who was advising government on urban air quality issues and we recognized that there might be future problems associated with the increasing uptake of diesel passenger cars," said Harrison, according to The Independent.

The documentary will also show how drivers are exposed to higher levels of diesel pollutants than cyclists and pedestrians.

"When people are in cars if they've got windows closed and the air conditioning on, they probably think that they are actually immune from the emissions from the vehicles in front of them and in reality that's not the case because the gases penetrate so easily that they will get into the cabin of the vehicle and depending on the ventilation of that cabin they may actually build up to much higher concentrations," said Professor Frank Kelly, Chair of the Committee for the Medical Effects of Air Pollution, according to The Independent.

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