Waiting times for HMRC are up to twice as long as the public body previously claimed, it has emerged.
The taxman had said people are only having to wait four minutes before being put through to an advisor, with HMRC hitting performance targets.
However it has now been revealed the body failed to include the time people spend listening to automated messages in the official figures.
The Government’s Public Accounts Committee claimed this could add an extra four minutes – doubling the time spent on hold before being able to speak to a advisor.
And the committee also found HMRC claimed calls were “successfully handled” if people hung up while listening to the automated messages.
Meg Hillier, a Labour MP and chairman of the committee, said: “There’s a difference between the way the beancounters at HMRC measure it and the reality for the taxpayer.
“It’s not honest and straightforward, they are treating the taxpayers like fools. HMRC are using figures that suit them but don’t reflect the reality of an actual wait.
“Long-winded automated messages are incredibly frustrating, especially for someone trying to file their tax return.
Waiting times on the HMRC hotline are up to twice as long as claimed
HM Revenue and Customs’s figures on hotline waiting times are incorrect
It’s not honest and straightforward, they are treating the taxpayers like fools. HMRC are using figures that suit them
“There should be short cuts to this, the phone line remains an important and vital way for people to talk to tax officials.”
The committee also warned the additional pressures of dealing with Brexit could have catastrophic consequences for HMRC, which is responsible for customs.
Ms Hillier said: “What was already a precarious high-wire act is now being battered by the winds of Brexit, with potentially catastrophic consequences.
“Action arising from allegations in the so-called Paradise Papers could also significantly increase the authority’s workload.
The taxman had said people are only having to wait four minutes
“HMRC accepts something has to give and it now faces difficult decisions on how best to use its limited resources – decisions that must give full consideration to the needs of all taxpayers.”
A HMRC spokesperson said: “We have improved our customer service standards enormously over the last two years. Our phone call handling has got better, with the average response time falling from 12 minutes in 2015-16 to below four minutes in 2016-17. We are determined to improve further.
“Time in the automated telephony system is valuable because it ensures customers are directed to the right person to deal with their question, or get their question answered through the automated system. The NAO themselves last year recognised, a large number of organisations use the same measures as HMRC.”
As well as failing on meeting waiting time targets, HMRC also failed to achieve its cost-cutting targets in the first year of the transformation programme because of “aggressive” assumptions.
HMRC currently expects to deliver £707m in savings
The agency currently expects to deliver £707million in savings through the programme by 2020, against a target of £717m – a £10m shortfall.
Frank Haskew, head of the tax faculty of the ICAEW, an accountancy body, said the timetable for the transformation programme had been too ambitious.
He added: “There is serious concern that the new demands on HMRC could be the straw that breaks the camel’s back.”
An HMRC spokesperson said: “We are fully focused on making the UK’s exit from the EU a success. The Customs Declaration Service (CDS) is on track for delivery by January 2019 and has the capacity to deal with a significant increase in customs declarations at the border.”