The government has little idea what the public sector spends on IT.
The disclosure comes from Martin Read, a former chief executive of computer services supplier LogicaCMG, who is leading a review at the Treasury into the costs of IT and back-office administration.
One official estimate is £13bn a year, but another is £21bn.
Coming from the private sector to work for government, Read was struck by "how little is known about what we actually spend".
He told the Government IT 09 conference at the Queen Elizabeth conference centre in Westminster, London, last week, "What did surprise me was just how little detailed information was kept about what was getting spent and what it was getting spent on.
"That generates a big problem because if it is hard to identify what the costs of back-office and IT operations are it means it is very difficult to establish trends to make comparisons between different organisations and with comparable situations in the private sector."
He added, "If something does not get measured very well, and is not visible, it is certainly not going to get managed very well. I think that is one of the issues we have got in looking at back-office spend and IT across the spectrum of the public sector."
Kable produces figures on IT spend, as does the government's CIO Council and other organisations. "We are not measuring like for like. It is a pretty messy science but our best estimate is that it is a spend of about £16bn a year. And it could be low as £13bn and as high as £21bn," said Read.
But the uncertainties could be removed. The plan is for statistics to be collected "on an exactly comparable basis in an auditable form and in a transparent way so they are available for comparisons".
This will enable officials to focus on "how an organisation is doing compared with last year" and will allow "comparisons with similar organisations, in this drive towards greater operational efficiency".
One aim of Read's review is to identify how much can be saved by simplifying and standardising systems and working practices. The results of the review are expected to be included in the government's 2009 budget.