13 January 2016
While overall spending fell dramatically after 2010, costs are increasing again and are likely to continue rising in the coming years, the NAO says.
The report highlights structural problems in the civil service, the union says, with departments unable to plan staffing strategically because they are forced to meet arbitrary spending cuts targets to a political timetable.
It adds that departments should start from the assumption that using their own staff would be the best use of resources, but there was little evidence they had "rigorously assessed the availability of skills internally or had compared in-house and external costs".
The auditors also found no evidence in any of its three case study departments of formal evaluation of the performance of, or feedback to, suppliers.
PCS general secretary Mark Serwotka said: "It is no surprise to find the civil service still does not have the use of external consultants under control because departments are being forced to make arbitrary and excessive spending cuts to a political timetable."