The retractable seats at the London Stadium were the main reason for the project going £51million over budget, the London Assembly has been told

The explanation came from London Legacy Development Corporation (LLDC) chief executive David Goldstone, who was questioned by the assembly’s budget monitoring sub-committee on Wednesday.

The meeting came about after leaks in November revealed that the cost of transforming the former Olympic Stadium into a multipurpose venue had ballooned by £51m from a projected £272m to £323m - bringing the total cost of the stadium to some £752m.

Accounting for the £51m, Mr Goldstone said: “A chunk of it was to do with the very unfortunate failure of the seating contractor who had been appointed to introduce the seating system”.

The retractable seating enables the stadium to be used for pitch sports, athletics, and concerts, and issues around the collapse of the contractor cost £21m.

“We were left with a partially-installed system with a Rugby World Cup imminent and very large crowds and very high profile, so [we] had to step in ,” he said.

A further £14m of costs were part of a joint venture with Newham Council as E20, and spent on “discretionary enhancements” to make the stadium more commercially viable, such as a reinforced pitch and better walkways so concerts could be held, lighting for a range of different sports, and a big screen that can provide advertising revenue.

A further £12m of costs was “contingency spending”, which was budgeted for in mid-2015 when the £272m figure was announced, but was not declared during the announcement.

Mr Goldstone said it was omitted because revealing how much money the corporation had set aside would have weakened its negotiating position with contractors.

He went on to explain that the final £4m of the £51m was spent on “insurances and management.”

Mr Goldstone said: “I suppose we could have included [the £4m in the publicised figure] but actually when we announced the £272m we were answering a specific question which was the value of the contracts for the transformation, and that was the question we answered.”

Assembly Members raised further concerns about a possible £8m recurring annual spend that was mentioned in the leak.

Mr Goldstone explained that this figure refers to the cost of retracting the seats to switch between the different stadium uses, and could range from £300,000 to £8m.

“We will know firmly what the costs [of using the seating] are in the near future,” Mr Goldstone said.

He highlighted that the London Stadium “is proving itself a really successful venue for major events and last year we had more than a million spectators in”.