A key part of plans to deliver the 2022 Commonwealth Games in Birmingham is set to cost nearly eight times its original £2m price tag.
Relocating a bus depot to help realise the athletes village in Perry Barr is now likely to come with a new bill of £15.5m, city council papers reveal.
Opposition councillors say the huge and unexpected hike poses a financial risk for the Labour-run authority.
The council says it will probe how the sums could have been so wrong.
The total cost of the 11-day sporting event is £778m, with £184m set to be covered by Birmingham City Council and partners.
Among its responsibilities, the council is tasked with delivering the athletes village - the competition's main hub - and plans to make it part of a wider Perry Barr regeneration programme.
But the work also means an existing National Express depot on Oscott Road must make way.
The depot is expected to be relocated to largely council-owned land on Aston Lane - with a new, projected cost of £15.5m.
According to Tory councillors, the athletes village was already a "major risk" to the local authority's front-line services.
The group's deputy leader Ewan Mackey branded the depot's spiralling costs "extremely concerning", adding the council's "ham-fisted approach to budgetary management" was souring the Games.
An emergency report to the council's chief executive stated the original £2m budget for a new depot was based on "limited information" and "assumptions" from an external consultant - "inaccurate" costing that would be probed.
The £13.5m rise, the report added, would be managed within the "overall cash envelope", with non-relocation "seriously jeopardising" delivery of the village.
The report continued such failure could see the council unable to meet grant requirements; possibly having to foot more Games bills from its own pocket.
Using emergency protocol to bypass cabinet, the depot's relocation and funding was given the green light on 31 December to avoid, the report said, delays to work.