17 December 2015
The report notes the department only has partial information on how much tax it recovers from counter-fraud efforts.
The union also says the overall amount of tax lost to evasion and avoidance is likely to be much higher than HMRC's estimate, with tens of billions of pounds lost to our public finances every year.
Last month MPs on the public accounts committee criticised HMRC's "woefully inadequate" efforts to tackle tax evasion and raised fresh concerns about aggressive tax avoidance.
One week later, HMRC announced plans to close all but 17 of its 170 UK offices, which the union said would "pose a significant threat to the operation of HMRC, its service to the public and the working lives of staff".
The plans were not announced to parliament and HMRC is not proposing to hold a public consultation.
The union is supporting the shadow chancellor John McDonnell's review of HMRC's role and resources and says the department's ability to tackle tax fraud is central to this.
PCS general secretary Mark Serwotka said: "While HMRC vastly underestimates the amount lost to tax fraud, collecting even a fraction of what is due would change the debate about public spending overnight.
"It is essential that HMRC is given the resources to do its job but, faced with more budget cuts, the department is putting this at risk with plans to close almost of its offices and cut even more staff."