Daimler has announced a voluntary recall of more than 3 million Mercedes-Benz vehicles in Europe amid mounting questions over its diesel engines.

The automaker said Tuesday that it was offering European owners a service upgrade that would improve diesel emissions. It will cost the company around 220 million euros ($255 million).

"The public debate about diesel engines is creating uncertainty," CEO Dieter Zetsche said in a statement. "We have therefore decided on additional measures to reassure drivers of diesel cars and to strengthen confidence in diesel technology."

Customers will not be charged for the upgrade.

Daimler also announced that it had created a new line of diesel engines "whose exemplary emissions have been confirmed by measurements carried out by independent institutes." The new diesel engine would be introduced rapidly across the company's entire model portfolio, it added.

Daimler was summoned by the German government last week to appear before a commission after local media reported that prosecutors were investigating possible cheating on emissions tests.

German newspaper Sueddeutsche Zeitung reported that prosecutors were investigating two engines used in over 1 million cars sold in the U.S. and Europe.

The owner of Mercedes-Benz and Smart said then that it was cooperating fully with authorities, but declined to comment on specific accusations.

The commission that Daimler executives will appear before Thursday was established in 2015 to investigate Volkswagen's diesel emissions scandal.

Volkswagen admitted fitting as many as 11 million diesel vehicles worldwide with software that could cheat nitrogen oxide emissions tests.