Mayor of London Sadiq Khan has ordered an investigation into the finances of the London Stadium after being told the total cost of converting the arena for use by West Ham has soared by £51m more than previously revealed.
Mayor Khan announced his investigation after Sky News learned that the estimated annual cost of moving "retractable" seats, installed to improve the view for football, has risen to £8m, and that moving the seats back and forth for summer events including athletics could take a month.
The original estimate was it would cost just £300,000 and take only five days to move the seats at the end of the football season and another five to move them back after the summer. Engineers now estimate it could take 15 days to move the seats in each direction.
The seating issue threatens the viability of the stadium's summer schedule, which next summer includes concerts as well as athletics, and could even delay West Ham's return for the start of the new football season.
The increase, calculated by stadium operators in the last fortnight, takes the cost of converting the arena for football from £272m to £323m. This includes £309m of "comparable" costs, and £14m of "further enhancements", including expenditure on seating and stewarding.
It takes the total cost of the stadium to £752m, all of which has been met by the taxpayer apart from a £15m contribution from West Ham.
Khan is said to be deeply concerned at the cost increase and deputy mayor Jules Pipe told Sky News they have inherited "a mess" from previous mayor, Boris Johnson.
"Last year the previous administration told the public that the costs had risen in total to around £270m. The new Mayor has now been told that cost will be over £320m," Pipe said.
"Understandably Sadiq has ordered an investigation as to why those two figures are so wildly different. As the new administration at City Hall clearing up the mess of the previous administration, we need to make sure that the stadium runs on a sustainable basis and it is affordable to London."
Pipe said the investigation would look at all aspects of the stadium including the deal agreed with West Ham, but indicated there was little room for renegotiation.
"The previous administration did give a good deal to West Ham but it is a legally binding contract. It will be up to the review to decide how best to protect the purse of Londoners," he added.
Johnson agreed to West Ham's request to install retractable seating in order to improve the view for football, and it was intended to allow the stadium to be quickly and cheaply converted for athletics and other events.
While the seats are described as "retractable", in fact the job of moving the seats is largely manual. Rather than sliding on wheels or airbeds as at the Stade de France, the stands are effectively demountable structures that have to be broken up and shifted by hand.
The problem is complicated by the fact that the original seat contractor, Alto Seating Solutions, has gone into liquidation.
Warnings that retro-fitting the system would prove costly and problematic have proved accurate, and may impact on the stadium schedule.
A summer concert series is currently due to begin just three weeks after West Ham's last home game, with a Depeche Mode concert on June 3.
The World Championships meanwhile end on August 13, 2017, and while they are unlikely to be disrupted it could delay West Ham's first home game.
Their first two matches of the 2017-18 season are understood to have already been scheduled to be away from home, but the building work could prevent them playing at home until September.
The seating issue is the latest problem to beset the stadium which was the centrepiece of London 2012.
West Ham's move has been dogged by violence inside and outside the stadium, raising questions about its viability as a football venue.
Last week coins were thrown at Chelsea fans including children during an EFL Cup tie, and police made six arrests as they battled to keep supporters apart.
A proposed naming rights deal has also collapsed, with some blaming negative publicity arising from the violence.
The stadium's legacy has been problematic from the outset. The original plan of reducing it to a permanent 25,000 seat athletics arena was abandoned when it failed to attract any sporting tenants.
A £100m offer from West Ham back in 2008 was rejected by government, and the football solution, with a capacity of 60,000, was approved.
West Ham and Tottenham, who wanted to demolish it and build a football-only arena, competed for the tenancy with West Ham victorious in large part because they would tolerate the retention of the track.
In talks with Johnson and the London Legacy Development Corporation (LLDC) however they argued for a new lower tier of seating closer to the pitch to improve the view.
The existing lower tier of seats, initially intended as the only permanent part of the stadium, was dug out and the new seats retrofitted.