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".
Meanwhile the car company announced Matthias Mueller as its new chief executive. The former Porsche boss replaces Martin Winterkorn, who stepped down in the wake of the scandal that has rocked the automotive industry.
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".
Mr Winterkorn resigned over the 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.
Mr Mueller's appointment as his replacement was announced after the stock markets had closed so it is not possible to say exactly how investors will take the news, though it is fair to say his appointment was widely expected.
Volkswagen shares fell 4.3% to 108 euros (£80) today, meaning the carmaker has lost about a third of its value, or 26 billion euros (£19 billion), since the start of the crisis.
The Volkswagen board is also expected to dismiss high-level engineers and designers, though no further details have been released yet.
Mr Mueller, 62, formerly worked as VW's head product strategist, and is understood to have substantial backing among the company's 20-member supervisory board.
Analysts said his first tasks at VW will be to clean out its senior management team, restructure costs and drive sales.
Mr Mueller, who trained as a toolmaker before studying IT has worked for various parts of the VW empire since the 1970s, experience that was seen as one of the decisive factors for the top job.
Equally importantly, he enjoys the support of the Porsche-Piech family clan who own the majority of the voting rights in VW.
He is known for his laid-back style, often appearing at car shows in a casual jumper rather than the expensively-tailored suits favoured by fellow executives.
He has until recently joked that he was too old to run VW, which is the largest carmaker in the world by sales.
Under his leadership, Porsche has successfully added new models and is expected to boost sales to 200,000 vehicles this year, hitting a long-held target three years early.