Campaigners have welcomed the start of an investigation by London Mayor Sadiq Khan into a £50m rise in the cost of converting West Ham's London Stadium.
The outlay to modify the Olympic venue has risen from £272m to £323m.
The investigation will look into inherited issues, including the cost of moving retractable seating.
"This is a breakthrough in our long campaign to highlight the intolerable burden on taxpayers," read a statement from the Olympic Stadium Coalition.
"We have never objected to the idea of West Ham playing at the stadium; the question is simply one of how much the club should pay, and how much the taxpayer should be expected to fund.