London mayor Sadiq Khan has ordered an investigation into the finances of the Olympic Stadium after being told the total cost of converting the arena for use by West Ham has soared by £51m.
Mr Khan announced his investigation into the conversion cost - up from £272m to £323m - as Sky News learned the estimated annual outlay of moving "retractable" seats is one of the factors behind the rise.
The cost of the seats, installed to improve the view for football, has increased from an estimated £300,000 to £8m.
Engineers have said work to move them could take 15 days at the end of the football season and 15 days to put them back after the summer - three times as long as the five days initially predicted for each period.
The seating issue threatens the viability of the stadium's summer schedule, which includes concerts as well as athletics in 2017, and could even delay West Ham's return for the start of the new football season.
Mr Khan is said to be deeply concerned at the cost increase, with deputy mayor Jules Pipe saying they have inherited "a mess" from predecessor 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.
"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."
Converting the Olympic Stadium has proven far more expensive than originally thought. Image: Isabel Infantes EMPICS Entertainment
The rise in the cost of converting the arena for football 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.
Mr Johnson agreed to West Ham's request to install the seating to allow the stadium to be quickly and cheaply converted for athletics and other events.
While the seats are described as "retractable", the job of moving them is largely manual.
Rather than sliding on wheels or airbeds as at the Stade de France in Paris, the stands have to be broken up and shifted by hand.
The problem is complicated by the fact that 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.
Summer concerts are due to begin three weeks after West Ham's last home game, with Depeche Mode due to play their first UK stadium gig in 23 years there on 3 June.
West Ham's 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 a 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 largely because they would tolerate the retention of the track.
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