A US software vendor working on the £6bn IT overhaul of the NHS has been removed from a major part of the project.

IDX Systems was working with BT to supply its Carecast software to the NHS’s Southern Cluster, which covers Kent, Dorset and Surrey. Last week IDX announced it had agreed to end its contract with Fujitsu, the prime contractor on the project.

The NHS's Connecting for Health agency is overseeing the 10-year National Program for IT (NPFIT) which aims to update the health agency's operations so it can deliver better patient care and service.

It was not clear why Fujitsu ended the contract with IDX, and the company did not respond immediately to a request for comment.

IDX’s software will be replaced by Cerner’s Millennium health care application, although it will continue to work with BT in the London region.

The move follows a warning in March by Richard Granger, director general of the NPFIT, who said contractors failing to meet deadlines might be dropped from the project.

"It was always envisaged that there might be some change of suppliers," said an NHS spokesman. "Senior figures in Connecting for Health have publicly stated this on several occasions and this demonstrates that the robust contracts we have in place are working. We remain confident that the national program will be a success and will deliver very real benefits to patients and clinicians."

He said the move isn't expected to affect the overall project. According to a statement on the NHS Web site, "this change in the south provides the opportunity for BT and IDX to redouble their focus on the delivery of services to the NHS in London."

IDX CEO James Crook Jr predicted a deployment of Carecast to the London region later this year, and said the cost of extricating itself from the south cluster effort would run between £1m and £2m.

According to Tola Sargeant, an analyst at research firm Ovum, Fujitsu's move to sever connection with IDX is no surprise, after "some well-documented delays" to the rollout of Carecast. "BT has clearly had significant problems fitting IDX's software to the UK system, and we find it hard to believe that Fujitsu and Cerner won't face many of the same challenges," Sargeant said in a note regarding the project.