Accountants have signed an open letter urging Chancellor Jeremy Hunt to address HMRC's "unacceptably low" customer service levels in his upcoming Spring Budget.
The group of leading professional bodies say that Hunt should prioritise investment in HMRC to tackle severe delays and disruption experienced by taxpayers and their agents.
Businesses currently claiming repayments and reliefs frequently wait upwards of six months for HMRC to process their claims.
HMRC customer service staff numbers have reduced by 24% in the past five years and the tax authority, unable to keep up with demand, has occasionally closed its phone lines.
Earlier this year, the Public Accounts Committee claimed that £42 billion in taxes remain uncollected, the equivalent of paying 75% of NHS staff salaries.
According to the professional bodies, including the Association of Chartered Certified Accountants and Chartered Institute of Taxation, improving HMRC's customer service levels will help the Government meet its economic objectives and boost productivity.
The group wrote:
"The Budget presents you with an opportunity to properly invest in HMRC, with a focus on improving customer service and effectiveness, so it can help both improve public sector finances and boost productivity in the UK as a whole.
"We urge you to consider this as a top priority in your forthcoming statement."
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