Only six in ten taxpayers (61.3%) were able to reach HMRC by telephone in March 2023 after listening to automated messages and choosing the option to speak to an adviser.
The figures from HMRC's monthly performance report are part of a trend of worsening phone line experiences, with 64.8% of taxpayers connecting to an adviser in February, compared to 60.1% in January.
In November last year, HMRC managed to answer 78.4% of calls, while in March 2022, it answered 66.1%.
In March 2023, there were 4 million calls to HMRC compared to 4.1m in January, and 2.8m in November 2022.
There was also a fall in client satisfaction in March, with HMRC's phone, webchat and digital service rating dipping from 76.7% in February to 75.7%.
To reduce phone call numbers, HMRC has been advising taxpayers to seek help from the department's digital services.
HMRC also recently closed down its self-assessment helpline between 12 June and 3 September to weed out taxpayers making ‘non-essential' calls that can be answered through its other services, and to boost the number of taxpayers able to speak to an adviser over the phone.
Bruce Cartwright, ICAS chief executive, said:
"We and other professional bodies continue to urge the government to invest more in HMRC to make sure they have enough resources to deliver at least an adequate service."
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