Tens of thousands of taxpayers were left in limbo last week when the taxman failed to send them self-assessment tax calculations.

With just days to go before the 31 January deadline, some advisers estimated that as many as 500,000 people were still waiting for their calculations to be delivered.

Taxpayers who send self-assessment returns to HM Revenue & Customs before the end of September are sent a calculation of what they owe, saving them having to work out their liability.

Thousands of these filers had received reminders to pay their tax, but had not been issued with a payment calculation.

Before news of the self-assessment hiccup emerged, HMRC expressed optimism that this year’s filing deadline could be the smoothest yet. Increasing online filing uptake (4 million returns are expected online in 2008) was set to make the process easier to manage and administer.

The calculations issue will be an embarrassment and an early test for HMRC head Dave Hartnett.

‘HMRC has said it will correct the problem and have the calculations sent out, but with the deadline approaching this delay comes at a very busy time and does not look good,’ said PricewaterhouseCoopers tax partner John Whiting

The taxman apologised for the oversight but warned that taxpayers would still have to pay up on time.

‘Some statements due to be sent to customers in December 2007 were not issued. We are also aware that some of those customers have subsequently received one of our general payment reminders with a blank payment slip. We are very sorry for the inconvenience this may have caused you and/or your clients,’ the taxman said in a statement.

HMRC promised to have updated calculations sent out to all affected by the end of last week.

The glitch in sending out calculations comes as HMRC awaits the outcome of a Public and Commercial Services Union ballot on strike action planned for self-assessment deadline day. The ballot closed yesterday and a decision on whether to proceed with a strike was expected by the end of this week.

Should the strike go ahead, tens of thousands of HMRC staff will not report for work on the busiest tax day of the year, possibly causing massive disruption to the processing of self-assessment returns.