Audi admitted 2.1 million of its diesel cars worldwide were fitted with the same software that allowed VW to cheat on US emission tests.

Some 1.42 million Audi vehicles with so-called EU5 engines are affected in Western Europe, with 577,000 in Germany and almost 13,000 in the United States, a spokesman for Ingolstadt-based Audi said on Monday.

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

The news came as German prosecutors opened an investigation against the former Volkswagen chief executive, Martin Winterkorn. The inquiry is focussing on "allegations of fraud in the sale of cars with manipulated emissions data," prosecutors said today.

VW sparked outrage last week after admitting that 11 million of its diesel cars were fitted with defeat devices that could determine when the engine was being tested and change the car’s performance for better results.

The scandal tarnished the ‘made in Germany’ brand exposing it to 18 billion dollars in US fines while its shares fell almost 30 per cent last week.

Matthias Mueller, former chairman of Porsche, replaced  Martin Winterkorn as VW chief executive on Friday.

Mr Winterkorn quit last Wednesday saying he had no knowledge of manipulation of emissions results.

Volkswagen shares continued to slide on Monday falling to nearly 7 per cent.