The price tag for upgrading the IT infrastructure of the UK National Health Service (NHS) could be three to five times higher than the original estimate when the project – billed as the world's largest IT installation – was launched two years ago.

The estimated 10-year cost (2003-2013) of the NHS' National Programme for IT (NPfIT) now stands at between £15bn and £30bn, according to a Department of Health statement. The government's previous 10-year estimate for the project was £6.2bn.

Along with new local IT infrastructures and a system for transmitting prescriptions electronically, the NPfIT includes plans for a database of electronic health records for 50m patients in England, accessible by 30,000 doctors and handling five billion transactions a year by 2008.

Also in the IT plan is a program called Choose and Book, designed to allow patients to make hospital appointments online from a choice of locations.

In a speech delivered in Birmingham in June, Richard Granger, director general of NHS IT, said the procurement program was on time and on budget. He added that "tougher negotiations with suppliers" had already saved the NHS around £6bn.

But two months later, the National Audit Office, the department in charge of independently auditing government departments, announced that it had launched an investigation into the cost of the project, which it hopes to publish in the third quarter of 2005.

In its statement, the Department of Health said that "significant financial benefits will accrue" from the IT project, pointing out that "it is generally accepted in the IT industry that implementation costs are some three- to five-times the cost of procurements and this is reflected in the business case that was made" for the NHS IT project.

A spokeswoman from the Department of Health declined say where the additional funding would come from or if local health authorities will be expected to help pay for the IT system.