More than four million calls to the taxman are going unanswered, new figures show, as HM Revenue & Customs concedes the problem is almost twice as bad as previously disclosed.

More than one in 10 callers to HMRC fails to get through to anyone, compared with just over one in 20 a year ago, prompting criticism from MPs that the service is letting down millions of self-employed people.

The true scale of the problem is even worse than HMRC is prepared to admit, as its audit ignores taxpayers who get an engaged tone when they dial the tax advice helpline.

HMRC’s own figures also reveal that 14 per cent of calls took more than 10 minutes to answer, though that does not include the time people spend navigating its automated call handling service, which can add an extra four minutes to the length of a call.

In September, the month before the deadline for paper tax returns, one in five calls took more than 10 minutes to answer.

It comes despite HMRC’s claims that it has improved its performance following previous criticism of its tax advice telephone service.