HMRC have begun issuing businesses with P9s showing the new tax code they must operate for some employees from 6 April.
The online notices are being sent out until 23 February 2009.
Agents who use the Revenue website to view P9s are advised that the seven-day window to follow the most recent tax codes will start from the date on which the taxman loads the notices into the data provisioning service.
Recipient firms will not receive a code number for every employee. The P9X on HMRC's 2009 employer CD-ROM will explain how to change the tax codes of other staff members in time for the start of the new tax year.