"We mishandled the situation in so far as our engines behaved differently within the testing regime to the real world. That's why we need to fix the cars, that's why we need to get the customers in, and that's why we need to put the cars right. We mishandled the situation, without a shadow of a doubt.

"It seems that in the test regime the engine behaved differently to the real-world situation via software. The software affected the flow of gas to the engine which reduced the NOx."

Willis said VW reacted "as quickly as possible"

The session at the House of Commons was the first opportunity for the transport select committee - a cross-party body made up of 11 MPs - to question VW representatives or the car industry on the dieselgate scandal.

Willis came under particular pressure on the amount of time taken by VW UK to remove affected vehicles - around 4000 cars still on the market - from sale. He argued that the firm had acted as quickly as possible, saying, "On 22nd September we were made aware by VW headquarters in Wolfsburg that there was a potential problem with diesel engines. On 28th September we were getting more details and I phoned Mr McLoughlin [Patrick McLoughin, the Transport Secretary] to say to him that as soon as I knew which vehicles were affected, I would voluntarily stop selling those cars. That phone call took place at 3pm on 28th September.

"On 30th September, at 9am in the morning, I received the VIN numbers from the various different factories. And this is the point: there were eight days between when we first knew it affected Europe and when I stopped selling cars. And the reason for that is the complexity of the number of cars involved. There are 60 different models, five different brands, three different engines and two different transmissions.

"I found the VIN numbers out precisely at 9am, and at 1.30pm, once I had clarified it with the computer systems, I stopped selling those cars voluntarily. It took four hours and 30 minutes from the time I knew the affected cars until I took action - four hours and 30 minutes."

Existing test comes under fire from MPs

Both Willis and the chief executive of the Society of Motor Manufacturers and Traders, Mike Hawes, were asked if the current emissions testing procedure was fit for purpose. Hawes admitted that the difference between real-world figures and those provided by the NEDC could be "up to 30%". "These figures are meant for a comparison process and always contain a disclaimer about the real world and this is for some very sensible comparitive reasons," he said.

"The test cycle dates back to the early 1980s and the industry recognises it isn't fit for purpose," he added. "When you put a vehicle on the test cycle, for instance, all of the electronic devices that you would turn on in the real world - air-conditioning, heating, sat-nav, Bluetooth - all of that has to be switched off, by law, because when the regulation was created that sort of technology wasn't available. Then there's the issue of real-world driving conditions - congestion, temperature, load, gradient - all huge and all ruled out by the test cycle to get a repeatable cycle."