Tax bosses are under fire to improve the public help service for people filling out their tax returns after new research found major flaws with the helpline.
New research by consumer group Which? has found that 29 out of 100 calls in a sample were cut off by a "very busy" automated message.
The pressure group found that the system's voice recognition software failed to understand a number of more complex phrases and questions.
Callers also faced an average waiting time of 18 minutes to speak to someone, with one caller in the sample having to wait as long as 41 minutes.
HM Revenues & Customs has admitted that the service "isn't good enough" and that improvements would be made through the introduction of new technology.
A spokesman: "HMRC receives over 40 million calls a year, but we know that some of our customers can struggle to get through on our helplines at very busy times. This isn't good enough, and we are working hard to improve the range of services we provide.
"This year we are introducing new technology to help us answer more calls quicker at busy times, and we are improving the digital services we offer so that more customers can find all they need online.
The spokesman insisted that HMRC were aware of the difficulties faced by people trying to fill out their tax returns, insisting: "There is more to do, and we are committed to improving the service we offer all of our customers at all times, to help them find advice and support when they need it."
Which? executive director Richard Lloyd said: "With large numbers of people soon to be seeking help with their self-assessment tax return, we want to see HMRC doing more to monitor and improve their call-waiting times."
The closing date for the self-assessment tax return deadline is on 31 January.