Zaha Hadid Architects (ZHA) have claimed the Japan Sports Council are withholding payment for the studio’s design for the Japan National Stadium in Tokyo unless the architects give up the copyright to the designs and sign a confidentiality clause.

The revelations – confirmed to CLAD by the architects – mark the latest escalation in an increasingly bitter fallout between ZHA and the JSC over the decision to drop the architects from the 2020 Olympic stadium project in July 2015 over a dispute about the cost of the project.

Japanese architects Kengo Kuma have since been selected to design the 80,000 capacity stadium in Tokyo.

ZHA claims that the JSC has since sent it a revised agreement with a clause allowing the newly-selected design team to use any product of ZHA’s original work.

In a statement, the architects said: "We can confirm that we received and rejected a written request from the JSC to modify our existing contract to allow the transfer of the copyright of the detailed design for Japan National Stadium, owned by ZHA, in exchange for an overdue final payment.

“ZHA has also declined a request to sign an additional new contract clause requiring the design team to no longer provide information or comment on the project, in exchange for this outstanding payment being honoured.

“Since October, we have been seeking to finalise an outstanding payment with the JSC, which is for months of work carried out by a team of more than one hundred architects and engineers in Japan and the UK working for ZHA and many other companies.

“We welcome that the JSC has acknowledged the issue of the intellectual property for the fundamental and detailed elements of the stadium design. We hope that these matters can be quickly resolved."

In a further twist, ZHA has today (14 January) submitted a report to the JSC detailing what it claims are significant similarities between the structure, layout and numerous elements of their original design and Kengo Kuma’s new design.

The company said the document “will form the basis for the discussions we hope to resume shortly with the JSC to resolve the important issue of the use of valuable design work that is currently the copyright of ZHA and the original design team.”

After the decision to appoint Kengo Kuma in December, ZHA accused the Japanese authorities “and some of those from our own profession in Japan” of working together to make sure the original design was dropped.

In a statement released at the time, the studio said: “This shocking treatment of an international design and engineering team, as well as the respected Japanese design companies with whom we worked, was not about design or budget. In fact much of our two years of detailed work and the cost savings we recommended have been validated by the remarkable similarities of our original detailed stadium layout and our seating bowl configured with those of the [new] design.”

Amid the ongoing row, work on the National Stadium is expected to continue as planned. The new design will cost ¥149bn (£806m, US$1.2bn, €1.1bn) and is due to be completed by November 2019. It will be built by construction giant Taisei Corp.

Japanese prime minister Shinzo Abe said the new proposal was a “wonderful plan that meets criteria such as basic principles, construction period and cost”.