The cost of the European Central Bank's (ECB) new 45-floor headquarters will clear €1 billion according to a statement released by the bank to celebrate its "topping out ceremony."

At an opening even attended by Frankfurt's Mayor Peter Feldmann, ECB executive board member, Jorg Asmussen, said that the total cost of the twin tower skyscraper would be around €200 million higher than the original €850 million budget. He added that €530 million had already been spent on the work.

Building started in 2010 on the site of 1920s listed-building Grossmarkthalle, which had previously been used for Frankfurt's wholesale food market.

The building, which stands at 185 metres, consists of two polygonal office towers with 45 and 43 floors, respectively, connected by an atrium.

Described in the project plans as a "vertical city," the towers will bring the ECB's 2,600 staff together under the same roof.

Asmussen blamed the extra costs on unforeseen problems in converting the site, including late discoveries of contamination of the roof and insufficient steel support structures, as well as difficulties in finding a contractor to lead with the building work when the tender was originally put out in 2008.

He added that "the new premises will provide the ECB with a modern and functional headquarters, and I hope that they will be viewed by the people of Frankfurt, and beyond, as an enrichment of Frankfurt’s skyline and the landscape of Europe."

The construction is also running six months late, but the bank insists that it will be ready to open in January 2014.

Since its creation in 1998, the ECB has never owned its own home, instead renting its current "eurotower" headquarters as well as two other office blocks in the centre of Frankfurt.

The revelation comes at an awkward time for the ECB, which has been a chief player in the EU's response to the eurozone debt crisis.

Together with the European Commission and the IMF, the bank has formed part of the "Troika," demanding severe austerity measures from Greece, Ireland and Portugal in return for multi-billion euro bailouts. I

It has also played an active role in keeping government lending costs down through its bond-buying programmes.