21 January is the deadline for registering for the online system
People using the HMRC online system to file their tax returns have been warned of a mistake that may mislead them.
The system still lets taxpayers ask for any tax owed for 2007-08 to be paid via their new PAYE tax code for 2009-10.
However this option, for tax of up to £2,000, should have been closed down on 30 December 2008.
The Revenue said: "This option should be unavailable but it is presently available." It added it was investigating the problem "urgently".
"If the revenue are still offering this, even if it is a mistake, they should honour it," said Chas Roy-Chowdhury of the Association of Chartered Certified Accountants. (ACCA).
"These are among the most vulnerable among the self-assessment taxpayers as they probably do not have professional help," he added.
The HMRC has been using a high-profile advertising campaign involving the television personality Moira Stuart to encourage even more people to file their self-assessment tax returns online.
As part of this, the deadline for filing on paper was brought forward to the end of last October, while the deadline for online filings, and payment of any tax due, has been kept at 31 January.
One feature of the system is that anyone who filed before 30 December could ask for the tax they owed to be deducted from their income in the next tax year, along with their normal PAYE tax payments, so long as the tax was only up to £2,000.
However the rules say this option does not apply to tax returns filed after 30 December.
The misleading glitch applies only to the Revenue's own free online system.
Taxpayers and their accountants can chose to use commercial software, sold by computer and accountancy firms, and the mistake does not appear to have been replicated in any of these.
Anyone who wants to use the Revenue system and who has not registered to do so has very little time left.
They have to logon to the HMRC website and request a pin code, which will be sent to them by post, and the deadline for making this request is 21 January.