Ministers should "come clean" on when they were first told about the Volkswagen scandal, Labour's shadow transport secretary has said.
Lilian Greenwood claimed the response of the Department for Transport (DfT) was "unacceptable".
The action against Volkswagen in the US began with diesel emissions research by the International Council on Clean Transportation 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."
She accused the DfT of performing "a series of screeching ?u-turns" on the issue.
"After initially saying that only the European Commission could conduct an investigation, Patrick McLoughlin has been forced to backtrack, but after this latest revelation there can be little faith in the department's response.
"Poor air quality is a growing problem in our towns and cities but the Government is in chaos on this issue, and it is becoming increasingly clear that real leadership was needed on this issue at least a year ago," she said.
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 in the US said 482,000 of the German car-maker's 2009-15 models 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.
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.
Alexander Dobrindt, Germany's transport minister, said VW had confirmed the affected vehicles include cars with 1.6-litre and 2-litre diesel engines in Europe.
Campaign group Friends of the Earth welcomed the UK investigation and called for the provision of "all necessary resources" to carry out new tests "wholly independent of car companies".
James Thornton, chief executive of environmental organisation ClientEarth, said: "The public must know the full scale of the problem and urgent action must be taken to fix it."
Volkswagen chief executive Martin 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.