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.”