The£11m-a-year NHS intranet system, NHSnet, suffered a major failure at the beginning of last week, throwing many hospital and GP e-mail messages into chaos.

Around 80,000 outgoing e-mails were blocked from leaving the system after lightning hit BT's data centre at St Albans on 1 September, causing a massive power surge. Five days later the system was beginning to work again, but there were still 40,000 messages waiting to be sent. The system was not fully operational until 7 September.

Although there is a back-up system for such eventualities, a spokeswoman from the NHS Information Authority admitted it had 'not performed in the way the suppliers had expected it to, which is why there have been delays'.

'A full investigation is being held into how this happened and how such problems can be avoided. We do apologise to the service for any inconvenience caused, but we did our best to put it right, ' she told HSJ.

Alan Ball from the Brussels headquarters of BT Ignite Content Hosting, which is responsible for the St Albans centre, said the service had been restored early on 2 September.

'But it takes a certain time to get back to full processing of everything that is there, including backup data.'

He said the president of the company had flown in from the US to oversee a full investigation.

'It has never happened before and we do not ever want to have it happen again, ' he added. 'Things have not worked as they should.'

Incoming mail from senders outside NHSnet was not affected and it is understood that clinical results from pathology labs would not have been affected as they still tend to use the separate X400 system.

The most likely disruption would have been to trusts supplying performance indicators to regional offices, but a Department of Health spokesman played down the extent of the problems:

'Some 1.25 million messages are sent a day on NHSnet, so it is not as bad as we thought.'

He said the DoH was working with BT and Syntegra, which provides the service, to improve the situation.

In an NHS Executive progress report on NHSnet, published in January, the Project Connect group, which is looking to widen NHSnet to GPs, effectively admitted there were problems.

'For professionals across the NHS to be able to rely on this network there is a need for guaranteed service and performance levels, ' it said.