At least 2.1 million Audi vehicles worldwide were fitted with the software that allowed parent Volkswagen to cheat US emission tests, the company says.

Some 1.42 million Audi cars with so-called EU5 engines are affected in Western Europe, with 577,000 in Germany, and almost 13,000 in the US.

Affected models include the A1, A3, A4, A5, A6, TT, Q3 and Q5, an Audi spokesman told the Reuters news agency.

Separately, German prosecutors started a probe against VW’s former boss, Martin Winterkorn.

Martin Winterkorn will be investigated over “allegations of fraud in the sale of cars with manipulated emissions data,” German authorities said on September 28.

The scandal has badly tarnished VW’s name, left it exposed to up to $18 billion in US fines, and wiped a third off its stock market value in a week.

German authorities have demanded that VW set out a timeline by October 7 on how it will ensure its diesel cars meet national emission standards without using cheat technology.

There were widespread German media reports at the weekend that the government ignored warnings two years ago that VW was using the software.

However, a government spokesman denied this.