Her Majesty's Revenue & Customs is failing to answer nearly 50% of phone calls from customers trying to access information about the tax and benefits system.
Leaked figures also show that the tax office has also missed its targets for responding to postal correspondence.
HMRC has a vital role in collecting the taxes required to fund schools, hospitals and other lifeline public services.
However, a combination of budget cuts and job losses have put pressure on a service that deals with millions of queries every year.
According to the Public and Commercial Services Union, which represents many of the workers in HMRC, around 34,667 full-time equivalent staff will have been lost since 2005.
The union estimates that the job cull amounts to nearly half of all HMRC posts in the last ten years.
It was also revealed earlier this month that HMRC's budget would be reduced by another £80million.
Sources close to HMRC believe the cuts are having a major effect on services provided to the public.
HMRC has an internal target of "handling" - which means answering - 80% of telephone calls from customers this year.
However, figures obtained by the Sunday Herald show that the year-to-date rate is only 51.2%
It is understood only 54.5% of calls were answered in April.
Similarly, the targets for responding to letters through the post are also being missed.
HMRC tasked its workers with clearing 80% of pay-as-you-earn and self-assessment queries by post within fifteen working days.
The figures show that the year-to-date-figure is 22.1%.
The same target is set for postal enquiries by employers and issues relating to national insurance contributions, but the year-to-date delivery rate is 31.8%.
An HMRC source said the body's plan to consider axing automatic penalties for late returns of self-assessment forms was partly due to a desire to cut down on paperwork.
The leaked figures also come after reports that the average time people spent in a phone queue to HMRC was 10 minutes and 53 seconds - more than double the wait from a year ago.
It was also claimed that 34.5% of calls were cut off.
Colin Borland, the Federation of Small Businesses Scotland's head of external affairs, said: "When we said we wanted less paperwork from HMRC, this wasn't quite what we meant. These statistics clearly demonstrate that paper correspondence isn't being dealt with in a timely manner.
"We appreciate that public funding is tight and we welcome moves to simplify the tax system and make it easier to do more online. However, when firms who often just want to stay on the right side of the law need to contact the tax authorities by post, they deserve a prompt service."
An HMRC spokesperson said: "This is always a busy time of year for us and there will always be peaks in demand leading up to key deadlines. But we're sorry that some customers are struggling to get through and we are working hard to improve our service. On 18 June we were able to answer more than 66% of calls overall and 72% of calls on tax credits, so things are getting better."
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