Fiat's plan to combine Chrysler with Opel is unhealthy, according to Volkswagen chairman Ferdinand Piech.

Piech told Bloomberg News at the VW Polo media launch in Sardinia: "Three sick people in one bed don't make one healthy person.

"I'm glad whenever the ones that are less viable help each other," he added.

Asked is a car company combining Fiat, Chrysler and GM Europe would be able to surpass Volkswagen, Piech said: "You cannot expect a world record when three footsore people try to run a marathon."

GM said yesterday (11 May) would choose its preferred partner for Opel before the end of the month.

Apart from Fiat, Magna International and a consortium of Russian investors, sovereign wealth funds from the Middle East have expressed interest in buying a stake in Opel and Vauxhall, its UK sister company.

GM has said it was prepared to be a minority shareholder in any new company.