Deutsche Börse has toughened its Dax admission rules to prevent companies that do not conduct the majority of their business in Germany from being included in its stock indices, as Frankfurt increasingly attracts listings from overseas.

A spokeswoman for exchange said: "The new rule is intended to ensure that companies that are included in the Dax indices reflect the German market. Other indices also do this. For example, the S&P 500 doesn't include companies that do not have a clear national character."