German auto giant Volkswagen will foot the bill for any additional taxes owed by customers who bought its vehicles that pollute more than originally claimed, its chief executive wrote in a letter to EU ministers Friday.

Matthias Mueller's letter came three days after VW revealed that internal probes found that carbon emissions levels for around 800,000 cars have been understated -- an admission that carries serious financial consequences.

This is because tackling CO2 -- a greenhouse gas blamed for climate change -- is becoming a rising priority in many countries, especially in Europe, where cars are often taxed according to their carbon emission levels.

"The Volkswagen Group pledges to take charge of any possible additional taxes," Mueller wrote in the letter seen by AFP.

"Volkswagen will inform tax authorities in all countries of the correct CO2-levels as they become available," he added, urging the authorities to send any bills for any additional levies directly to VW, and "not to our clients".

German Transport Minister Alexander Dobrindt said earlier this week that customers should not have to cough up additional levies because VW deliberately understated the vehicles' CO2 emissions.

The latest revelation is yet another setback for VW, which sank into the deepest scandal in his history over its admission in September that 11 million of its vehicles are equipped with devices aimed at cheating official pollution tests.

The so-called defeat devices switch pollution controls on when the car is undergoing tests and off when it's back on the road, allowing the vehicles to spew out harmful levels of nitrogen oxide.

VW has lost nearly 40 percent in market capitalisation over the scandal which has sparked investigations around the world.

Shares in the company closed down 0.33 percent at 97.18 euros.