The Institute of Chartered Accountants in England and Wales (ICAEW) is calling for an emergency taskforce to tackle HMRC's underperformance.
In a statement, the ICAEW said that ministers must act immediately to address long-standing delays at HMRC, which have become a drag on the UK's economic growth.
According to the professional body, many taxpayers and agents have to wait up to a year for HMRC to respond to their queries made via phone or post.
Meanwhile, callers are reporting having to wait for more than an hour before being cut off without speaking to an official.
The backlog in queries can be partly attributed to HMRC customer service staffing levels recently falling from 25,500 to 19,500 people.
The ICAEW says the proposed taskforce would focus on identifying problem areas to help end the "unacceptable" delays and would also recommend improvements to HMRC's service standards to enable business growth.
Michael Izza, chief executive at the ICAEW, said:
"HMRC performs a vital function supporting businesses and growth.
"Despite this vital role and the commitment from hardworking civil servants, HMRC is often overlooked and overstretched, leading to problems, and we would be pleased to work with it to tackle these issues."
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