HMRC has denied running a "deliberately poor" phone service in an attempt to push taxpayers to seek help online instead.
Nearly 44,000 customers were cut off without warning after being on hold for more than an hour last year, a report by a committee of MPs has found.
It said HMRC's service had got even worse since its last report and warned the authority it had "damaged trust in the tax system".
HMRC chief executive Jim Harra said the committee's claims on its customer service were "completely baseless" and added "we've made huge improvements to our service standards, with call wait times down by 17 minutes since April last year".
The report comes ahead of the deadline for self-assessment tax returns on 31 January, which could lead to increased demand for help.
HMRC's phone line went dead on 43,690 customers who had been waiting 70 minutes to reach an adviser in the first 11 months of the 2023-24 financial year, the Public Accounts Committee (PAC) report said.
The number for the year so far is more than six times the 6,875 for the whole 2022–23 financial year.
This was because HMRC's system could not cope with the volume of calls, but customers were not warned they were about to be cut off, nor were they called back, the report added.
The figure for the number of callers cut off was published by the National Audit Office (NAO) in May last year, but MPs have highlighted it among fears HMRC was running down its own helpline.
Claire told the BBC she was cut off while waiting to get through to HMRC on the phone last year.
She said she then sent two letters with proof of postage but was told both times they were "lost" after weeks of waiting. She claimed the problem was only resolved once she sent another letter that had to be signed for.
She added: "It was very, very difficult. If you're elderly like my parents you'd be stuffed. If I got it wrong, I'd be the one that's prosecuted."
Sir Geoffrey Clifton-Brown MP, chair of the PAC, said HMRC was "excavating its way to new lows" in its customer service every year.
He added: "Worse, it seems to be degrading its own services as a matter of policy."
Speaking to BBC Breakfast on Wednesday, he said that HMRC was "still very slow at moving" to online communication compared to other government organisations, "like the passport agency".
He added: "If they don't provide the digital services, which they don't in every case… they can't be surprised that their customers want to contact them by phone."
The committee has called for "bold and ambitious leadership" to improve its customer services, and better tackle tax system abuse and unpaid debts.
In 2023-24, HMRC wrote off £5bn in debts as uncollectable, up from £3.2bn in 2022-23.
The report called for the authority to get a better understanding of the offshore tax gap – the difference between how much tax should be paid and what was actually paid.
It also raised concerns over decreasing rates of criminal investigation and prosecution for tax-related offences.
The recommendations come after a series of criticisms levelled at HMRC, since it started to work towards becoming a "digital-first" organisation in 2010.
In March last year, it announced its phone line would be closed between April and September, but was forced to reverse its decision within 24 hours.
And in May a report found that customers were waiting an average of nearly 23 minutes to get through to an adviser.
Mr Harra, first permanent secretary and chief executive at HMRC, said: "We will always be there to answer the phone for those who need extra help. At the same time, more than 80 per cent of customers are satisfied with our digital services, with more and more people using them to quickly and easily manage their tax affairs."
HMRC said the phone lines and webchat will be open as usual but customers may experience longer wait times.
PCS members will take part in eight weeks of industrial actions in the hope of reinstating the reps.
The former Harry Potter star is ordered to pay the sum after losing a tax appeal.
David Crisp told undercover agent he had ignored "government edicts" to sell £1.7m of his luxury Boadicea the Victorious product.
The PCS union believes the workers were sacked for their role in strike action last year.
Copyright 2025 BBC. All rights reserved. The BBC is not responsible for the content of external sites. Read about our approach to external linking.
