The annual total amount of time taxpayers spent on the phone waiting to speak to a HM Revenue and Customs adviser has more than doubled to the equivalent of almost 800 years, according to a report by Whitehall’s spending watchdog.

The National Audit Office (NAO) found that average call waiting times at HMRC have soared by more than 350% in five years, with increasing numbers of people not getting through in the first place or having their calls terminated, according to an official report that says the public is being “let down”.

It said the quality of customer service provided by HMRC had been “far below” the levels expected in recent years, and that its phone lines in particular were “not delivering”.

Taxpayers cumulatively spent the equivalent of 798 years (7m hours) waiting to speak to an adviser in 2022-23 – more than double the 3.2m hours spent waiting in 2019-20.

The NAO said HMRC’s performance when it came to its phone lines “has continued to decline”, with the department answering only two-thirds (67.2%) of callers’ attempts to speak to an adviser during the first 11 months of the 2023-24 tax year. This was down from 77% two years earlier, and is well below the target of 85%.

Those who did get through to an adviser waited an average of almost 23 minutes in 2023-24 – up sharply from an average of five minutes in 2018-19.

Meanwhile, the report revealed that more taxpayers waiting to speak to someone are having their calls ended by HMRC if the department believes it is the sort of query that can be dealt with online instead.

The tax office is trying to cope with demand by encouraging more customers to turn to its digital services first so that queries can be resolved quickly and easily online. But “fiscal drag” had pulled more people into the tax system, while individuals’ tax affairs were getting more complex – for example, because they did more than one job or worked as a freelancer – the report said.

Digital services were better suited to straightforward queries and reporting changes in customers’ circumstances, the authors said.

The report comes less than three months after similarly scathing findings from another official body, parliament’s public accounts committee. The findings may have helped prompt the announcement by the Treasury this week of £51m in funding “so HMRC staff can answer more calls and help customers over the phone”.

Gareth Davies, the head of the NAO, said: “HMRC’s telephone and correspondence services have been below its target service levels for too long.”

He added that while many of its digital services worked well, “they have not made enough of a difference to customers, some of whom have been caught in a declining spiral of service pressures and cuts”.

The consumer body Which? said the report “paints a sorry picture of a service in serious decline”, while the TaxPayers’ Alliance claimed the situation would improve if ministers ordered civil servants who had been allowed to work from home to get “back behind their desks”.

In response, HMRC accepted that customer service standards relating to its phone lines were “still not where we want them to be”, but added: “We’re making strong progress in our efforts to improve our customer service, and additional funding has been confirmed by the government this week.”

A spokesperson said: “Millions more people used our highly rated online services last year, saving them waiting on the phone and freeing up our advisers to deal with those people who need extra support.”