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More than a quarter of phone calls to HM Revenue and Customs went unanswered in the last year, figures have shown.

Of the 64.7 million calls made by taxpayers between April 2014 and March 2015, 27.5% - 17.8 million - were either unanswered or resulted in a busy tone.

HMRC's chief executive apologised for the figures and said the revenue's service had not been "up to scratch".

The service has pledged to invest £45m in about 3,000 customer services staff.

Another 2,000 staff will be moved temporarily from within the HMRC to help with the tax credits deadline and letters and forms, it said.

HMRC set a target to answer 80% of calls.

But showed that in some months only about two in three (65.5%) of phone calls were answered.

In September 20.8% of people heard busy tones and could not join a phone queue when they called, while 13.7% of calls were not answered.

In total 7.2 million calls made to the HMRC last year - 11% of all calls - ended with people hearing a busy tone.

Lin Homer, HMRC chief executive, said: "Despite our best efforts, our call performance hasn't been up to scratch and we apologise to all those customers who have struggled to get through to us."

Ms Homer said the HMRC had already invested in new telephone equipment and online services.

The new £45m investment will come from current HMRC funding rather than from additional revenue from the Treasury, the HMRC added.