Former VW chief executive Martin Winterkorn arrives at the German parliament on Thursday © Getty

The former chief executive of Volkswagen on Thursday insisted he only found out about the illegal software at the centre of the German carmaker’s diesel emissions scandal in September 2015.

Appearing before a German parliamentary committee, Martin Winterkorn refused several times to answer when he first learned of the emissions cheating, citing ongoing investigations.

But pressed by MPs on whether he knew that defeat devices had been installed in VW diesel cars to improve emissions test results, Mr Winterkorn eventually replied: “Certainly not before September 2015.”

However, he expressed surprise that VW staff had not alerted him sooner to the problems. “It is hard to understand why I wasn’t apprised — clearly and in good time — about the . . . problems,” he told MPs.

It was Mr Winterkorn’s first public appearance since the diesel scandal broke. He led VW from 2007 until his resignation in September 2015 — soon after the company admitted to US regulators it had installed illegal software dubbed a defeat device in its diesel cars that served to understate emissions of harmful nitrogen oxides in official tests.

German prosecutors are investigating the scandal and its origins, and whether senior VW executives including Mr Winterkorn failed to tell investors about the affair quickly enough. US authorities are also investigating individuals caught up in the scandal.

Some VW investors are separately seeking damages in legal actions after suffering losses when the company’s shares plunged in value shortly after the affair became public.

Mr Winterkorn spent almost 35 years at VW and had a reputation as a stickler for detail who was obsessed with the quality of the company’s products. He once boasted in an interview that he knew “every screw in our cars”.

MPs said they found it difficult to square that reputation with the man who appeared before them on Thursday.

"I find it implausible that he didn't know about defeat devices before September 2015,” said Oliver Krischer, a Green MP and committee member. “This is someone who was a details-oriented CEO, who cared about all technical aspects of the business and constantly projected that image to the public.”

Herbert Behrens, the committee chairman and a socialist MP from the opposition Left Party, said he thought Mr Winterkorn “held back what he really knows” about the scandal.

Much of the questioning by MPs revolved around what Mr Winterkorn was told about the diesel affair, and when.

Court documents in the US released last week said VW employees involved in the cheat software briefed “executive management” at the company’s Wolfsburg headquarters in July 2015, although no executives who allegedly participated in the meeting were named.

VW said in a statement last March that Mr Winterkorn was sent a memorandum in May 2014 about a research study suggesting some of the company’s US diesel cars were emitting far higher levels of NOx when tested on the road compared to in the laboratory. VW added at the time it was not clear whether he took notice of the memo.

Mr Winterkorn was asked about the memo by MPs on Thursday, but declined to answer.

Mr Winterkorn said he had no contact with the German government about the scandal until after the cheating was made public by the US authorities.

He said shortly after the US Environmental Protection Agency revealed VW’s wrongdoing on September 18 2015 he had met the transport minister Alexander Dobrindt and told him that VW not only had a problem in the US but in the rest of the world.

He subsequently phoned German chancellor Angela Merkel. On September 23 he resigned, a step he described as “the hardest thing I’ve ever had to do”.

In a statement before the question-and-answer session with MPs, Mr Winterkorn appeared to blame colleagues for not informing him of VW’s diesel affair.

He said he was a person who “appreciated honesty”, and in his 35 years at the company had spoken to employees on an almost daily basis. “Unlike what you might read in the papers, there was no reign of terror” while he was chief executive, he added. “I never had the impression that people shied away from having a frank talk with me, or from telling me unpleasant things,” he said.

Mr Winterkorn said he would “never have thought it possible” that millions of VW customers would be cheated in this way. But he added that he would have to live with the fact that his name would be forever closely associated with the diesel affair.

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