Total costs on the project have risen to over £700m

The mayor of London has ordered an inquiry into the conversion of the Olympic Stadium after it was revealed costs have soared by a further £51m to over £320m.

The total cost of building the Olympic Stadium and subsequently turning it into football club West Ham’s new home has now tripled to over £700m from its initial estimate.

Of this £700m-plus, less than 3% is being paid for by the private sector. West Ham is paying just £2.5m in rent per year to the public sector.

The price of retractable seating for the stadium to enable it to quickly convert from a football stadium to athletics venue alone has risen from an estimated £300,000 to £8m, according to reports, and was one of the main factors that triggered the mayor’s decision.

The London Legacy Development Corporation (LLDC), which has overseen and commissioned the stadium’s conversion, awarded the £154m contract to Balfour Beatty, which had to rebuild large parts of the stadiums structure, as well as redo foundations and piling in order to bear the strain of supporting the largest cantilevered roof in the world.

The LLDC’s current chairman David Edmonds speaking to Building in the summer admitted the conversion cost was “considerable”.

He added: “The roof cost a lot more than we thought, because once we started to do the designs for the cantilevered roof, we realised that more strengthening had to go in than originally expected. Secondly, the development and evolution of the retractable seating has also been more expensive. I don’t attempt to hide the fact it has cost a lot more than originally planned.”