HM Revenue & Customs could boost tax revenues by up to £100 million a year by providing more support to professional tax agents, a report suggests.
The National Audit Office (NAO) said tax returns filed by agents on behalf of individuals are more likely to under-declare the amount of tax owed than returns filed by the taxpayer themselves.
It said while the under-declarations result for a number of reasons, such as errors, a failure to take reasonable care and evasion, a key reason agents seem to make more mistakes is likely to be because they are dealing with individuals whose tax affairs are more complex.
As a result, it said providing better support for tax agents and targeting ones who regularly make mistakes could boost revenues by up to £100 million a year.
But the NAO said a lack of data on individual tax agents prevents HMRC from taking a more tailored approach with agents or providing them with feedback.
It said if HMRC made better use of the data it had, it should be able to make more targeted interventions.
Amyas Morse, head of the National Audit Office, said: "HMRC could achieve better value for money by increasing the level of tax compliance of taxpayers who are represented by tax agents, while doing more to minimise the costs of its engagement with such agents.
"The department has a strategy for working more closely with tax agents but implementation will require a plan detailing how the expected benefits will be achieved.
"To the extent that there are opportunities to increase tax revenues significantly, it may make sense to incur some short term set-up costs."
HMRC has taken a number of steps to develop its relationship with agents, including setting up a priority telephone line for agents with questions on self-assessment income tax and pay as you earn.