Self-employed tax credit claimants should let HMRC know their total profits for 2009/10 by 31 January 2011.
Failing to provide the correct details could lead to an overpayment. HMRC will be sending letters urging claimants to contact the department as soon as they know their final income figure for the previous tax year.
As part of a separate Government initiative, in October, HMRC and the Department for Work and Pensions published a strategy designed to tackle error and fraud in benefits and credits.