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‘defeat systems’, even after news broke of the so-called 'dieselgate' scandal in the US.

News that VW did not immediately suspend sales of cars with defeat devices will compound the German car giant's problems.

The firm halted the sale of 4,000 new cars on Thursday.

They are thought to be fitted with the same defeat software which US authorities claim has been used to give false readings in emissions tests.

A VW spokesman said the firm "regrets" consumers had purchased the cars and would now need to have them fixed. Those in the process of buying an affected VW will be able to pull out of the deal and select another VW product.

The 4,000 cars affected – which amounts to around three per cent of the group's UK stock – include VW, Audi, Skoda and Seat models. These cars are all fitted with the EA 189 EU5 engine – the same unit found in 1.2m vehicles that VW recalled last Wednesday.

Cars fitted with the EU6 engine will continue to be sold.

US authorities say that the VW diesel cars affected may have emitted 40 times the safe level of controlled particles.

The scandal has re-invigorated the campaign against diesels as being detrimental to our health. However, car firms have defended the engine type.

Carlos Ghosn, chief executive of Renault-Nissan and president of Acea, the European car industry's trade body, said that there had not been "full scale cheating" across the automotive industry and that the entire sector should not be punished as a result of the VW scandal.

Mike Hawes, chief executive of the UK's Society of Motor Manufacturer and Traders, said: “Consumers are right to be concerned following the events of the past 10 days. But we must remember, the actions of one company do not mean collusion. Implicating other brands or companies would be unfair and wrong.”