The firm says a Freedom of Information Act request has shown that HMRC handled a total 81,066 complaints in 2015-16. This is over 7,000 more than the previous year and an increase of more than 16,000 on 2013-14. 

In 2008-09, at the height of the financial crisis HMRC dealt with 83,917 complaints.

Saffery Champness’ analysis also shows that the volume of complaints upheld has increased steadily. The firm says this may be due to improved systems and processes uncovering errors and anomalies; alternatively HMRC may be looking to tie up matters swiftly and avoid lengthy and complex appeals for its overstretched staff.

Most recently, in 2015-16, a third of complaints (33.95%) were fully upheld, compared to a quarter (25.05%) in 2012-13. Overall, half of all complaints made (50.84%) were either fully or partially upheld in 2015-16, compared to 37.12% four years previously.

James Hender, partner and head of the private wealth group at Saffery Champness, said: ‘This data reinforces the anecdotal impression that many of us in the industry have been receiving. HMRC is making a large number of errors each year and has bitten off more than it can chew.

‘Our clients are acutely aware that HMRC can have a poor response time and that it does not have sufficient resources to enable it to deal effectively with the more complex investigations. The suspicion will be that frontline staff are being dragged away from their day jobs to deal with the ever growing mountain of complaints received.’

Saffery Champness points out that the seven-year complaint high comes in the wake of HMRC’s launch in December 2015 MTD plans, which have already caused concern among business and individuals and which will require more regular contact with HMRC.

Lucy Brennan, partner at Saffery Champness, said: ‘Over three million taxpayers used an online personal tax account this year and, with HMRC aiming to go fully digital by 2020, more and more people are coming into contact with the new digital services and are inevitably coming up against obstacles.

‘Ironically, there is also the possibility that in seeking such large quantities of data, the administrative costs of the rising number of complaints and investigations will cost HMRC more, rather than raising additional revenue more efficiently.’

HMRC said in a statement: 'We apologise to the customers that we have let down. We take complaints very seriously, we want people to tell us when we make mistakes or our service falls short so we can improve our systems and services.'