More than 100,000 e-mails were held up when problems with a firewall on 1 September severed NHSnet's link to the Internet.
The latest problem to affect the NHS private network will disappoint the National Health Service Information Authority, which has made significant efforts to improve the performance of the troubled service over the past two years.
BT Syntegra, which operates the NHS Messaging Service, told CW360: "A power failure at our data centre caused the firewall to fail and it had to be rebuilt. We switched to a back-up system but this took longer than expected to deal with the NHS messaging service.
"The problem with NHSnet was given priority during the crisis, and as a result we were able to ensure that no messages were lost, although there was a backlog. We have some lessons to learn from this incident and we will be reviewing our back-up system."
The NHSnet initiative, a cornerstone of the government's 1998 White Paper on Information for Health, was intended to ensure all GPs were online by the end of 1999.
Delays and ongoing concerns over security meant that the target date for 100%-connection was revised. It now stands at April 2003.
A successful and secure electronic messaging system is central to many key NHS projects, including the push for electronic patient-referrals in a bid to shorten waiting times for cancer patients requiring treatment.