The inquiry committee is trying to find out whether EU institutions and politicians could have done more to prevent car manufacturers from cheating on emissions tests.

The MEPs have requested appearances from four former commissioners and the two current commissioners in charge of industry and environment.

Verheugen is the only one to have rejected the invitation.

“I fully respect the role and the responsibility of the EP, and I appreciate your explanation why the committee wants to hear me,” Verheugen wrote.

However, he told Van Brempt he would not come.

“I still do not see how I could contribute more than the commission, which is in possession of all relevant documents," he wrote.

"Therefore I ask your understanding that my position remains unchanged."

The letter was dated 19 May, but Van Brempt told this website she had not seen it on 24 May, when she told this website that the “rude” commissioner had not replied.

She told this website on Wednesday (1 June) the rejection was “unacceptable”.

The pressure on Verheugen, who currently works as a consultant, is increasing.

On 26 May, EP president Martin Schulz wrote a letter, also seen by this website, telling Verheugen he was “surprised” by the rejection, and reminding him that a key regulation that deals with illegal cheating software in cars was adopted during his tenure.

Schulz stressed that former commissioners hold valuable insights and that “democratic accountability” also applies to them.

Both Schulz and Verheugen are German and of the centre-left Social Democratic party, which sits in the EP with Van Brempt's socialist group.

Copies of the letter, written in German, were sent to Van Brempt, and to current EU commission chief Jean-Claude Juncker.

Verheugen's appearance before the committee, if he caves into the pressure, is scheduled for 14 July.