London Mayor Sadiq Khan is inviting independent companies to bid for the right to investigate how conversion costs at the London Stadium were allowed to spiral by £51 million since 2015.
Khan ordered the investigation in November when it was revealed that the bill to taxpayers for converting the centrepiece of the London 2012 Olympic and Paralympic Games into a multi-purpose venue had climbed to £323m.
Last month, the London Legacy Development Corporation (LLDC), the body that operates the stadium for the Greater London Authority, admitted that problems with the venue's retractable seating were the biggest cause of the rise in costs.
Premier League side West Ham, who have a 99-year lease on the 60,000-seat stadium, made a one-off contribution of £15m to the conversion costs and pay £2.5m a year in rent. The total price tag for the stadium, which also has a new roof, is estimated to be £752m.
In a press release, Khan said: "There are some huge questions that need to be answered about the financing of the London Stadium. We need to find out how on earth the transformation costs were allowed to skyrocket, and whether appropriate checks were made before key decisions were made.
"But just as important in this process will be looking to the future to ensure we get the stadium into a situation where we are able to reduce its cost to the taxpayer and it can operate as a successful multi-purpose stadium that our city can be proud of."
As well as West Ham games, the stadium is set to host the world athletics and para-athletics championships this summer, and music concerts by Depeche Mode, Guns 'N' Roses and Robbie Williams.
When questioned by the London Assembly's budget-monitoring sub-committee in December, LLDC chief executive David Goldstone said "a chunk" of the £51m increase was due to the contractor hired to build the retractable seating going bust shortly before the 2015 Rugby World Cup.
Goldstone said resolving this issue so the venue could stage Rugby World Cup games and still be ready for West Ham's move to the stadium this season cost £21m. The remaining £30m was spent on other unforeseen improvements to the stadium that were needed, including a large screen and lighting system.
Goldstone, who announced his resignation in November, added: "The £323m, that is the final number. The work is finished."
Khan, however, has been unhappy about the situation since taking office in May and has blamed his predecessor Boris Johnson for mismanaging the project.
On Friday he outlined the areas on which he wants the investigation to focus, with a decision on which company has won the contract expected next month. Those areas include:
:: What role the International Olympic Committee, London 2012 organising committee and other sports bodies played in designing the stadium and what thought was given to its use post-Games
:: The construction, financial and operational deals that were made between the public authorities and private contractors, and who was ultimately responsible for them
:: What due diligence, negotiation and research was done to ensure tax-payers got value for money, and the stadium's ongoing financial viability
West Ham's first season at the venue has been a decidedly mixed bag as the club has enjoyed a significant increase in attendances -- up 20,000 a game compared to Upton Park -- but crowd trouble and complaints about a lack of atmosphere have been the main topics of conversation among fans and pundits.
The venue has also staged international rugby league and motor racing, with plans to host Major League Baseball and Twenty20 cricket under discussion.