A financial report to the force’s Strategic Police and Crime Board this week revealed the rising budget.

The force had set aside £30.6m to fund construction of two “superblocks” in Oldbury and Perry Barr.

The report stated: “More detailed work in relation to the custody programme has identified that despite further value engineering work the costs of building the two new custody blocks is forecast to be higher than the allocated budget.

“The reason for the increase in costs is predominantly due to inflationary increases in the cost of building resulting from a more buoyant market than when the original cost estimates were undertaken.

“It is therefore necessary for the custody build scheme budget to be increased from £30.6m to £34.6m.

“The risk of further cost increases has been reduced significantly , but not totally removed, because both sites have now been purchased and fixed price contracts are being used for the building works.”

Construction information specialist Glenigan has named Willmott Dixon as winning contractor for the scheme.