The shock increase to £15.5 million ($20.4 million/€18.2 million) is detailed in a report to the interim chief executive of Labour-run Birmingham City Council.

According to Birmingham Live, the report, called "Commonwealth Games - National Express Bus Depot re-location", says the original allocation was based on "limited information and a number of assumptions and exclusions".

"The increase of £13.5 million is a result of full details being provided, and costs being re-based to align with development timescales," it adds.

Conservative Group members claim the news could even threaten the project to build the Athletes' Village for the multi-sport event, due to take place from July 27 to August 7, 2022.

Birmingham City Council told Birmingham Live in a statement that the plan would go before a full Cabinet meeting for approval next month.

"A revised full business case (FBC) is being prepared reflecting the details outlined in the bus depot report and other changes to the programme," the statement reads.

"This will be presented to Cabinet for approval in February 2020.

"All costs will be managed within the overall cash envelope approved through the revised FBC."

Conservative Group members have also criticised Birmingham City Council for detailing the increase in a report published on New Year's Eve "without a formal Cabinet meeting and without the opportunity for a 'call-in' to scrutinise the decision".

Councillor Ewan Mackey, deputy leader of the Conservative Group, said: "Concerns are growing for the Athletes' Village that the Council are due to provide for the Commonwealth Games after it emerged that the required re-location for the National Express Depot would now cost the Council nearly eight times the original budgeted cost.

"The Conservative Group has criticised the Labour-run Executive for by-passing the usual decision making process to implement this decision between Christmas and New Year."

The Athletes' Village is the biggest construction project for the event, according to Birmingham 2022.

The Village is expected to house 6,500 athletes and officials during the Commonwealth Games, before being converted into 1,400 homes for the community as part of a long-term regeneration plan for Perry Barr and the surrounding areas.

Eventually, 5,000 new homes will be constructed.

In June of last year, it was claimed by the United Kingdom Government that Birmingham and the West Midlands region would benefit from a £778 million ($1 billion/€913 million) investment to stage the 2022 Commonwealth Games.