Three of Germany's biggest companies are facing a fraud investigation related to their work at Berlin's troubled new airport project, already five years behind schedule, a newspaper reported Sunday.

Siemens, Bosch and Deutsche Telekom unit T-Systems are suspected of submitting inflated bills to the company building the BER airport on the outskirts of the German capital, Bild am Sonntag said.

The report, citing internal documents, said that the anti-corruption unit of the consortium responsible for the new hub had launched a probe into large payments to the three companies approved by top BER executives.

The bills in question date from before the planned 2012 opening, and were requests for additional payments for originally unforeseen services. Officials now hope BER will be operational in late 2017.

Bild said that large invoices submitted by the companies for subsequent services were paid "often without question".

Lawyers for the airport consortium called the rate at which the additional bills were paid "unparallelled and suspicious", according to Bild.

Bosch and T-Systems declined to comment on the report but Bild quoted an airport consortium spokesman as saying that "all payments made since the start of the project will be subject to another overall review".

A spokesman for Siemens declined to comment on the report but noted that the company had introduced broad internal review practices in recent years.

"We cannot completely rule out wrongdoing by individuals but if there are indications that the law and internal guidelines were violated we will decisively investigate this in cooperation with the relevant authorities," he told news agency DPA.  

The German capital's new international air hub has become a multi-billion-euro planning disaster and a running joke for many Berliners, tarnishing Germany's reputation for engineering prowess, efficiency and punctuality.

After several delays and budget blow-outs due to serious technical flaws -- especially in the fire safety and smoke extraction system -- officials now hope to open BER in two years' time.

But even that date has looked to be in serious doubt after recent setbacks including the bankruptcy of the German division of Dutch group Royal Imtech, which is handling major electrical, ventilation and plumbing work at the site.