HM Revenue & Customs (HMRC) has failed to halt the widespread use of "highly contrived" tax avoidance schemes, according to a National Audit Office (NAO) report.
HMRC has 41,000 open avoidance cases on its books and the schemes are thought to cost the exchequer £10bn in lost tax.
The NAO said while the tax advance disclosure regime - brought in in 2004 - has helped the department make some important headway avoidance schemes are still being used by large numbers of taxpayers.
It said in each of the last four years, more than 100 new avoidance schemes have been disclosed to HMRC.
"While HMRC believes most of these would be defeated if tested in the courts, there is no evidence that their usage is reducing." the report added.
Some 30,000 users of 'partnership loss' schemes and disguised remuneration have been identified .
"While HMRC believes most of these would be defeated if tested in the courts, there is no evidence that their usage is reducing," added the report from the government's spending quango.
Amyas Morse, head of the National Audit Office, said: "HMRC must push harder to find an effective way to tackle the promoters and users of the most aggressive tax avoidance schemes. Though its disclosure regime has helped to change the market, it has had little impact on the persistent use of highly contrived schemes which deprives the public purse of billions of pounds.
"It is inherently difficult to stop tax avoidance as it is not illegal. But HMRC needs to demonstrate how it is going to reduce the 41,000 avoidance cases it currently has open."
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