The state-of-the-art transport hub for the industrial region, home to giants like Mercedes maker Daimler, was scheduled to be completed in 2021, but the board member told Reuters this had now been pushed back to December 2024.

Deutsche Bahn said last month the project was 24 months behind schedule, despite efforts to speed up work in recent months. A spokesman for the state rail operator declined to comment on Wednesday, saying the topic would be on the agenda of a supervisory board meeting on Dec. 13.

The costs of the project, which are being shared by Deutsche Bahn, the local and federal governments, and Stuttgart Airport, are now expected to exceed 7.5 billion euros, compared with a previous budget of 6.5 billion, the supervisory board member said, speaking on condition of anonymity.

The backers set up a project management company to handle the Stuttgart train station project, but the major contractors involved include Strabag's Ed. Zueblin and construction group Hochtief.

Businesses and the International Monetary Fund have warned that decaying infrastructure could become a drag on growth in Germany without increased investment, but a number of major projects have been hit by delays.

A planned new airport for the capital Berlin is also years behind schedule and still has no fixed opening date, although owners hope construction can be finished next year.

While also delayed by three years, Hamburg's Elbe Philharmonic concert hall did successfully open this year.

News of the ballooning costs of the Stuttgart project comes as Germany's leading political parties struggle to form a new government, more than two months after national elections.

(Reporting by Markus Wacket; Writing by Maria Sheahan; Editing by Alexander Smith)

Stocks treated in this article : Hochtief, STRABAG AG