Self-assessment tax forms should be simplified, a powerful group of MPs has said.
The Commons Public Accounts Committee has highlighted problems with the forms, saying they are too complex and lead to people making mistakes and subsequently either under- or overpaying their tax. When errors are discovered, some taxpayers can be fined for filling out the wrong amount.
In addition, the committee said HM Revenue & Customs should improve its advice call centre and make its website easier to navigate.
"Too many people are unintentionally making mistakes, resulting in an estimated underpayment of tax each year of some £330m," said Edward Leigh, head of the committee.
He added: "The department is falling short of industry standards in how quickly phone enquiries are picked up and dealt with. Too many callers are still being shunted round the system rather than directed straightaway to the staff with the right knowledge."
However, an HMRC spokesman pointed out that reform was under way and that a shorter self-assessment form had been launched for the 2007-08 tax year.