Chartered Institute of Taxation warns that a new HM Revenue and Customs pay as you earn system is sending out the wrong tax code to people.
The CIOT said that the wrong information unless corrected by people may reduce employee's pay packets by over £1000 a year.
HMRC said that anyone who has changed jobs over the past couple of years would be more likely to have received a duplicate or inaccurate coding notice, as its new system failed to account for the change in jobs and assumes that workers have multiple jobs.
People with more than one job normally have their personal allowance, which is not taxed, counted against the job with the highest wage.
Personal allowances stand at £6,475 for most people under 65 for the coming tax year.
