Cuts to taxpayer helplines could see standards of service plunge back to "unacceptable" levels, MPs have warned.
The Commons Public Accounts Committee (PAC) has expressed concern about plans by Her Majesty's Revenue and Customs (HMRC) to reduce spending on the services by a third over the next five years.
MPs warned that helpline services "collapsed" in 2014 - 2015 due to HMRC underestimating the demand for telephone assistance when it reduced available staff by 5,600.
The PAC report said HMRC admitted its service levels were "unacceptable" at times, and in October 2015, the average wait to have a call answered reached almost 35 minutes.
More than a quarter of callers, 28%, simply gave-up trying to get through in 2015-2016, and charges for callers spiralled with every £1 saved by HMRC cutbacks resulting in additional costs of £4 for people trying to get through.
Callers had to wait a total of four million hours to get help in 2015 - 2016, the report revealed.
PAC chairwoman Meg Hillier said the prospect of further cuts would "chill the blood" of people needing assistance, particularly those on low incomes.
"The prospect of HMRC making further cuts to spending on customer service will chill the blood of many taxpayers.
"HMRC's recent performance in this area has been appalling for long periods and left members of the public counting the cost in time and money.
"It is bad enough that people trying to pay their fair share of tax should have been kept waiting for so long.
"But holding for HMRC's helpline has hit them in the pocket too - a serious concern for those on low incomes and a dismal message to send to small businesses, the self-employed and anyone else simply seeking advice.
"HMRC has serious work to do before this committee is confident it can provide a consistent, efficient service that properly meets the needs of taxpayers and optimises tax revenue.
"Efforts to meet Government spending targets must not come through ill-conceived measures that effectively penalise the people departments are intended to serve," Ms Hillier said.
The report found that customer service levels "collapsed" between 2014 to 2015 as average waiting times tripled, and only saw an improvement towards the end of last year after the recruitment of an extra 2,400 staff.
MPs have expressed fears that planned cuts of 34% by 2021, as HMRC moves to rely more on digitalised systems, will cause another major drop in standards.
HMRC is now aiming to answer 90% of calls, and expects the average waiting time to fall below five minutes, the PAC reported.
MPs called on HMRC to examine what impact its "poor level" of service was having on tax revenue.
An HMRC spokesman said: "This is an inaccurate, out of date reflection of our phone performance. We acknowledge that service levels in the early part of last year were not acceptable and we apologised at the time.
"But the PAC is well aware our phone lines have since fully recovered and we are now offering our best service levels in years. Wait times are now below five minutes and customers consistently rate the support they receive on the phones as excellent. New online services also mean people can self-serve 24/7 or contact us via webchat or Twitter. There's never been a more convenient service for our customers."
Public and Commercial Services union general secretary Mark Serwotka said: "The department must now halt the reckless plans it has to slash more jobs and close all but a handful of its UK offices, and the Government must give HMRC the resources it needs, or we will just see repeats of these problems for years to come."
PAC also warned HMRC still had important decisions to take on its planned major new IT system.