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The union says HM Revenue and Customs must now fully commit to ensuring none of its call centres are handed to private companies.
HMRC has published the results of a trial, which ran from February 2012 to February 2013, using two private suppliers in call centres dealing with tax credits enquiries.
The department's analysis shows the private companies missed targets they were set and underperformed compared to their public sector counterparts.
Despite regular claims privatisation makes services more efficient and effective, HMRC found the suppliers were no cheaper.
The report says the private companies, which undercut HMRC wages for its employees on the trial, struggled to retain staff throughout and found the work was more complex than they were used to or expected.
PCS general secretary Mark Serwotka said: "This supports what we have said all along that privatisation is not the solution, particularly when dealing with such complex and detailed advice to taxpayers.
"We are pleased HMRC has said it has no plans to do another trial but it must now fully commit to focusing on investing in its own staff to provide the very best services it can."