Stevenage’s Lister Hospital is still experiencing ‘major computer problems’ following Friday’s cyber attack in which a virus was introduced onto IT systems at NHS trusts, businesses and other organisations across the UK and globally.

The virus – a form of malware – froze computer screens and displayed a message saying they could not be re-set until a ransom was paid.

The East and North Herts NHS Trust, which runs the Lister, was among those affected.

This morning the NHS trust posted a message on its website saying its IT specialists were working hard to resolve the problem as quickly as possible.

It said patients with appointments today should go to the hospital on Coreys Mill Lane as normal unless told otherwise.

The NHS trust is not providing a service for non-urgent blood tests at the Lister or the New QEII Hospital in Welwyn Garden City.

The phone lines are up and running again and patients can call the switchboard on 01438 314333, but it is not yet operating at full capacity – so people are urged to be patient when calling this number.

The NHS trust’s diabetic eye screening service isn’t currently running and you should only go to A&E if it is absolutely essential or you are concerned.

The hospital has also thanked patients for their patience and understanding.

In a separate statement, the East and North Hertfordshire Clinical Commissioning Group also thanked patients, as well as staff and volunteers.

Dr Nicky Williams, the commissioning group’s deputy clinical chairman, said: “I’d like to thank patients for their understanding at this difficult time. Please continue to use hospital services wisely.

“I’d also like to thank all the NHS staff and volunteers who have worked tirelessly throughout the weekend to get systems back up and running as quickly as possible.”

NHS England has said there is a “complex emerging picture” about the impact of the attack.

There were widespread fears this morning that the attack may continue to spread as millions of people switched their computers on after the weekend.

Seven trusts out of 47 that were hit on Friday are still facing serious issues.

The virus that hit the NHS is known as Wanna Decryptor or WannaCry and has infected at least 200,000 machines in 150 countries since Friday.