Police are investigating a forged letter warning of security breaches at a Norfolk hospital.

Health chiefs last night moved to reassure patients that their records have not been compromised.

Police were called in after it emerged that a forged letter purporting to come from Nerissa Vaughan, chief executive of the Queen Elizabeth Hospital, in King's Lynn, was being circulated.

'The chief executive has confirmed that the letter is a hoax,' a QEH spokesman said.

'In view of the fact that similar hoaxes have been perpetrated in the past we have informed the police.

'This is now being treated as a criminal investigation and, therefore, we cannot make any further comment.'

The document – on a letterhead similar to Ms Vaughan's – warned that it would not be possible to tell whether e-mails from other NHS organisations or GP surgeries were genuine until an overhaul had taken place.

'Due to recent breaches of our security system I have ordered a complete overhaul and maintenance of our e-mail server to take place on Saturday 5 and Sunday, February 6, 2011,' the letter added. 'All contact with the media is also being strictly monitored for possible interference.'

But the hospital spokesman also told the EDP: 'Our patient information systems remain secure and have never been compromised.

'We constantly monitor the performance of our computer network and the shutdown of our e-mail system over the weekend of February 5 – 6 is part of the planned routine maintenance carried out on a regular basis to maintain computer traffic speeds.'

Staff at the 492-bed hospital were informed of the e-mail shut-down last week. Its 1,600 computers were given added security after malware attacks in 2006.

Norfolk police said: 'Norfolk Constabulary can confirm that the Queen Elizabeth Hospital has contacted officers in relation to a series of hoax communications and the matter is being investigated.'