The Government has denied claims the UK agreed to delay the introduction of tougher car emission limits as part of a deal to support Germany's motor industry.
The allegations were made by a former Lib Dem transport minister, Norman Baker, who said Prime Minister David Cameron made the decision following a personal request from German chancellor, Angela Merkel.
A Department for Transport spokesman said: "Negotiations on new car CO2 targets are an entirely different issue to manufacturers illegally falsifying emissions tests.
"The UK has one of the most aggressive and comprehensive programmes to reduce C02 from cars anywhere in the world, backed by £500m in investment. The UK is a global leader in the move to low carbon transport.
Earlier, the department had described the claims a “distortion of the correspondence.”
The comments came as the fallout from the Volkswagen emissions scandals continues.
Switzerland banned the sale of VW cars caught up in the row.
The ruling affects diesel models containing "defeat devices" – software able to change the engine's performance and run cleaner during official testing.
As well as VW models which fall under the "euro 5" emissions category, other brands in the company's group, such as Seat and Skoda, are affected.
However, the ban has been restricted to vehicles that are yet to be sold or registered, meaning Swiss motorists who currently own an affected car will not need to take action.
New VW cars with "euro 6" engines are also immune from the ban, as they contain an updated emissions system.
Meanwhile, it has emerged EU officials knew devices able to skew exhaust readings were being used five years ago and the European Commission was warned about their dangers two years ago.
The devices were banned by European legislation in 2007, making it a legal responsibility for the national testing authorities to enforce the ban - indicating Germany failed to uphold EU law.
A 2013 report by the European Commission's Joint Research Centre warned of loopholes in the legislation and urged new road testing laws to prevent cheating by car manufacturers.
On Friday, Matthias Mueller was named as the new boss of Volkswagen – moving from his former role as the chief executive of Porsche - and vowed to restore trust in the car-maker.
He replaced Martin Winterkorn, who resigned this week after it was revealed VW had fitted 11 million cars and vans with defeat devices.
Germany's transport minister said VW has manipulated test results for about 2.8 million cars there, pointing to cheating on a bigger scale than initially thought. A number of VW employees have been put on leave until details are cleared up.
The UK Government has also begun its own investigation into the use of rigged data, which could see all diesel cars in Britain re-tested.
