Builders stealing iron from the building site of Cologne's new metro line may have contributed to the collapse of surrounding buildings, a local newspaper reports today.

Citing documents from the investigation into the metro disaster, the Kölner Stadt-Anzeiger reports that iron to strengthen the metro walls was sold on to scrap dealers.

The city's transport company has not ruled out a link between this finding and the partial collapse of the metro last year, which reduced the city archives and a number of neighbouring buildings to rubble and killed two people.

The construction of Cologne's new metro line has been beset by massive delays and spiralling costs for years.