The National Audit Office is conducting an investigation into HMRC's cost reduction plans.

The NAO says the probe will consider whether HMRC is 'well-placed to achieve value for money from the programme given the progress to date and the wider context of change within the department'.

It follows in the wake of HMRC's recently announced plans to spend up to £34m and recruit 1,000 more staff to ramp up the performance of its call centres and slash customer waiting times.

Earlier this year it emerged that taxpayers have to wait almost three times as long to contact HMRC than they did two years ago. It takes, on average, just over four minutes to get through to an adviser on the phone, compared to one minute and 31 seconds in 2010.

Chief executive Lin Homer, vowed that the extra investment will ensure the department will hit its target of answering 90% of all calls by the end of March 2013, some two years earlier than anticipated.

HMRC currently has 17 UK contact centres employing 8,500 staff, handling around 60m calls a year. Call centre response rates are running at 74% of all calls in 2011/12, up from 48% in 2010/11.

An additional £9m will be spent this year, said Homer, and up to £25m in 2013-14 in order to cut call waiting times 'drastically' and create 'a much more responsive HMRC'.

Paul Aplin, ICAEW's Tax Faculty chairman and a member of the joint initiative on HMRC service delivery - dubbed the move 'a really encouraging step forward'.

Just last week, the NAO published its first review of the data systems used by HMRC to report its performance. While it passed them as 'fit for purpose' it recommended that the department consider more automated and cost-effective ways of collecting data.

The NAO's findings are set to be published before the end of the year.