The complicated design of Tokyo's new National Stadium meant only a limited number of contractors could tackle the project and was the biggest cause of its budget blowout, the Japan Sport Council revealed Tuesday.

Board members of the JSC, an external body of the sports ministry, said the skills needed to build special components of the design account for ¥76.5 billion of the project cost alone.

On June 29, the central government confirmed that the total construction cost for the stadium could jump to ¥252 billion, about ¥90 billion more than the ¥169 billion the council estimated last year.