The UK National Health Service has warned hundreds of thousands of staff that it has been forced to push back an email upgrade to Microsoft's Exchange 2007 to next year.

At least 300,000 staff at surgeries and hospitals nationwide have been told the highly anticipated upgrade will be delayed.

Due to start at the end of this month, the move from Mirapoint to Microsoft Exchange 2007 will mean NHS staff can access new features, including the ability to wirelessly synch mobile devices to access emails on the go.

In a letter to staff, Will Moss, program head for the affected internal email system, NHSmail, this week said the start date had been delayed from 27 September because the organisation could not yet guarantee the new service would match the standard of the existing system.

Doctors' union the British Medical Association (BMA) expressed dismay at the delay of another project by NHS IT body Connecting for Health but added it was important the upgrade was carried out with the minimum of disruption.

Moss wrote: "We have identified that some components require further testing and improvement to give us the assurance that the whole service will meet or exceed the current levels of performance. This additional performance and scalability testing will mean a deferral to the previously announced schedule."

"We believe that undergoing further testing is a prudent step to minimise risk, giving us and NHSmail users the confidence that the new service is absolutely robust and reliable."

A spokesman for the BMA said: "It is disappointing that another Connecting for Health project has been hit by delay, although it is important that the migration is got right to avoid any disruption to service.

"We hope that when the migration is complete it will provide an improved, reliable and secure service for NHS staff."

Contractor Cable & Wireless is now scheduled to begin the work on 9 January 2009.