UK ministers have been told to "come clean" on when they were first told about the Volkswagen scandal.
Labour's shadow transport secretary Lilian Greenwood claimed the response of the Department for Transport (DfT) was "unacceptable".
Environmental group Greenpeace wrote to the Government to ask if it knew about the cheating of emissions tests before this month.
The action against Volkswagen in the US began with diesel emissions research by the International Council on Clean Transportation (ICCT) in 2013 and 2014 which found a huge discrepancy in real-world performance and official laboratory tests.
Transport Secretary Patrick McLoughlin announced yesterday that diesel cars would be re-tested in the UK.
Ms Greenwood said: "Ministers must come clean and admit when they were first told about the diesel emissions scandal.
"?The International Council on Clean Transportation, the body which helped to expose the problem, warned a year ago that dangerously high levels of nitrogen oxide emissions were not confined to America.
"It is unacceptable that the Government waited this long to take action."
Greenpeace published figures which it claimed show manufacturers of diesel vehicles built to comply with European emissions standards spent up to £13.6 million lobbying EU politicians last year.
Its UK executive director, John Sauven, said: "It's time for our ministers to be completely transparent on what they knew and when about the pollution fix scandal.
"Many people will want to know which matters more to our government - the polluters' profits or the health of their citizens."
A DfT spokesman said the UK Government has been "at the forefront of action at a European level" to introduce updated emissions testing.
He added that the ICCT report published in October last year "did not identify the vehicles tested".
VW has admitted that 11 million vehicles worldwide were fitted with sophisticated software which conned testers in the US into believing their vehicles met environmental standards.
The Environmental Protection Agency said 482,000 of the German car-maker's 2009-15 models in the US were fitted with the defeat device to switch engines to a cleaner mode when they are undergoing official emissions testing.
Once on the road, the cars produced nitrogen oxide pollutants at up to 40 times the legal standard.
VW has not revealed whether cars in the UK are affected but Germany's transport minister, Alexander Dobrindt, said 2.8 million vehicles in his country have the software.
He added that VW vans as well as cars are caught up in the scandal.
Mr McLoughlin said on Thursday that the Government was taking the ''unacceptable actions of VW extremely seriously''.
He announced that the UK regulator would investigate and re-run laboratory tests and called on the EU to conduct a Europe-wide investigation to find out if cars have been fitted with defeat devices on the continent.
The British Lung Foundation urged the Government to commit to "routine, independent real-world testing on all cars".
Volkswagen chief executive Martin Winterkorn resigned over the emissions scandal but insisted he was not aware of "any wrongdoing on my part''.
According to reports the 68-year-old could receive a severance package of up to two years of his annual salary, which would be worth over £22 million.