Thousands of people could be paying too much tax because they cannot get through to HMRC on the phone, MPs warned last night.

The level of customer service provided by the tax office is now so unacceptably poor that it could be considered a ‘genuine threat to tax collection’, the Commons public accounts committee found.

The report also criticised HMRC for failing to prosecute wealthy tax evaders who hide their fortunes overseas.

HM Revenue and Customs answered only half of calls during the first half of 2015 – leaving millions seeking advice on their tax affairs unable to get through. It also managed to answer just 39 per cent of calls within five minutes last year.

Shambles: HMRC's customer service operation was savaged by a committee of MPs. Pictured is HMRC's headquarters in Whitehall

The committee said customer service had got worse since being described as ‘abysmal’ two years ago, leading to the risk that people may not be paying ‘the right tax’.

Committee chairman Meg Hillier told the Mail that the failings could have led to people paying more tax than they should have. She said: ‘It could be people paying not enough; it could be people paying too much. There are a lot of dropped calls; people giving up.

‘People ring up because they want to be guided through the system, but if they cannot get through to the right person, they can’t ask. So it could lead to underpaying or overpaying.’

The taxman has a target of answering 80 per cent of calls, but it is falling well below this. In 2014/15, HMRC had answered 72.5 per cent of calls to its contact centres, the report found – declining to 50 per cent over the first half of 2015.

The committee described HMRC’s standard of customer service as ‘unacceptable’.

Warning: Public accounts committee chairman Meg Hillier said HMRC's failings could have led to people paying too much tax

‘HMRC did not provide us with any indication of when or by how much its customer service would improve, beyond a vague aim to improve year on year,’ the MPs said.

‘It acknowledged that people are more likely to pay the right tax when they find HMRC easy to deal with, but, in the words of its own chief executive and permanent secretary, “we are still struggling”.

‘We are concerned that customer service levels are so bad that they are having an adverse impact on the collection of tax revenues.’

The report also described the number of prosecutions for offshore tax evasion as ‘woefully inadequate’ and cites HMRC’s failure to gather intelligence on losses through aggressive tax avoidance as an obstacle to improving British tax laws.

Miss Hillier said: ‘HMRC must do more to ensure all due tax is paid. The public purse is missing out and taxpayers expect and deserve better.’

In the highly critical report, the MPs said HMRC appeared to have ignored warnings by the committee that it needed to show it could deal ‘robustly’ with individuals and firms which deliberately misled it. An HMRC spokesman said it had collected a ‘record’ £517billion in revenues in 2014/15, and the gap between what was due and what was collected was one of the lowest in the world.

He added: ‘We explained to the committee that we hadn’t provided a consistent level of customer service in the first half of the year and we had recruited around 3,000 new staff to improve service levels. But these customer service issues did not affect our ability to collect tax.’

Mike Down, tax partner at accountants RSM, said: ‘The department’s record on call handling has been abysmal [but] getting calls answered more quickly is only half the battle. You also need contact centre staff with the wherewithal to be able to answer questions and give clear and unambiguous advice.’

Chas Roy-Chowdhury, of the Association of Chartered Certified Accountants, added: ‘Phoning HMRC is worse than going to the dentist – people dread it.

‘It’s time the Government put its hand in its pocket in order to get more staff to answer the phones and get the tax in.’

The Public and Commercial Services union said: ‘The department has cut too many staff and services are suffering.’