The NAO, which scrutinises public spending on behalf of Parliament, looked at how HMRC has rolled out online filing since 2006.
More than 11.5 million customers a year submit one or more tax returns to the Revenue’s “ambitious” online programme, the NAO said.
Filing tax returns online cuts processing, storage, stationery and postal costs and is in line with Government plans to expand digital services.
But predicted cumulative savings from online filing have been revised down, from £220m by 2011-2012 to £190m. HMRC estimates that expanding online filing has delivered £126m in savings so far. The report described HMRC’s online drive as a “real achievement”, being on track and on budget.
It continued: “HMRC does not, however, have sufficient understanding of the relative costs it incurs in online filing compared to filing of paper returns or of the costs and benefits to customers.
“For these reasons it is not possible to conclude whether benefits are being maximised and value for money has been achieved.
“Significant improvement is needed in these areas to drive future development on an informed basis.”
The report said that while HMRC has yet to measure whether anticipated costs and benefits to customers are being achieved, it plans to do so.
HMRC’s revised savings figure means it expects £190 million in cumulative savings to be made by next year, with annual savings of £66 million thereafter. The programme is expected to break even during 2012-2013.
The NAO found that take-up rates are “good and increasing” but HMRC has lowered some of its original forecasts.
By the end of 2010-11, take-up rates for PAYE (Pay As You Earn) in-year returns and self-assessment were 94% and 77% respectively, broadly in line with 2008 forecasts.
But the rates for VAT and corporation tax stood at 67% and 42% respectively, some 20% below 2008 forecasts.
In the light of actual rates achieved, HMRC lowered its original forecasts of 100% take-up by 2012 to around 97% by 2013-2014 for the three business taxes. The 2013-2014 forecast for self-assessment is 84%.
The report said: “Customers generally recognise the efficiencies and practical benefits online filing offers, although professional bodies have told us of their concerns about the cost and usability of filing VAT and corporation tax returns online.”
It found that while HMRC had achieved an “ambitious timetable”, there was some dissatisfaction with aspects of the service, including “repeated dissatisfaction” about delays in receiving log-in details to gain access to the self-assessment online service during the busiest periods.
The report said: “Levels of satisfaction with the assistance offered through the various helpdesks vary, and HMRC’s website does not currently meet accessibility standards in full.
“HMRC uses various methods to collect customer feedback but it has not benchmarked customer satisfaction with that of other online services.”
The Expansion of Online Filing of Tax Returns report said the Revenue service needs to develop a clearer strategy for how and when it expects to achieve universal filing online in the light of lower than expected take-up levels.
HMRC has delivered the programme within a reduced budget so far, with the final cost expected to be £332 million, against an original budget of £373 million.
Amyas Morse, head of the NAO, said: “The programme is largely complete, to time and budget, and more than 11 million customers are filing online.
“It is an integral part of the department’s drive to increase efficiency.
“However, HMRC cannot demonstrate that the benefits are being maximised. Significant improvement is needed in its understanding of costs and benefits to inform future development.”
A spokeswoman for HMRC said: “Our online services have enabled, this year alone, a record 6.9 million people to send their self-assessment tax returns to us over the internet.
“In addition, 93% of company returns and over three-quarters of all VAT returns have come in over the internet.
“In excess of 11 million taxpayers now take advantage of our online services, enjoying many benefits and saving the taxpayer £60 million a year.”