GCHQ, the electronic surveillance agency, has been criticised on social media for a poorly-timed tweet which was sent as the cyber-attack unfolded.

United Lincolnshire Hospitals Trust says all of its computer systems and many phone lines are switched off. 

Where possible, the trust will be diverting some emergency cases to local hospitals which are not affected.

The trust says:"We ask patients to only come to our A&Es if absolutely necessary. 

"We do not yet know how long this incident will last, and therefore are cancelling all outpatient, endoscopy, cardiology and radiology appointments scheduled for this weekend." 

Dr Qassid in Kettering emailed the BBC to say that the email system in Kettering General Hospital was shut down as a precaution.

He added: "I hope this attack doesn't lead to any breaches of confidentially of our patient records."

An anonymous hospital worker told the BBC: 

"The cyber-attacks have really left our team in a bit of a rut, can't access our internet or use our emails. Have had to put work on hold as I can't do anything. Not really what the NHS needs right now!"

The National Cyber Security Centre tweets...

Colchester General Hospital in Essex is asking patients not to come to its accident and emergency department and is postponing all non-urgent procedures for the rest of the day.

It's due to the cyber-attack on NHS IT systems across the UK.

Colchester is still open for 999 emergencies.

NHS Kernow CCG, Royal Cornwall Hospitals Trust and Cornwall Partnership Trust all say they are not affected by the cyber-attack.

North Tees and South Tees NHS Trusts, CCG in Hartlepool and Stockton also say they are unaffected.  

NHS Blackpool Clinical Commissioning Group have said patients should avoid contacting GPs or using hospital services "unless absolutely necessary", while staff deal with the effects of a cyber attack.

The Patients Association says the attack appears to have been "highly co-ordinated and aggressive".

"A police investigation will no doubt be required," it adds.

"Though today's may be the largest attack of this sort, it is not the first - yet the lessons from earlier incidents have not been learnt."

A member of staff at the Royal London Hospital, who did not wish to be named, has said staff have been warned not to use social media.

People have been reacting on Twitter to news of the cyber-attack.

Image source, @AmyGrimmers

Simon Enright, director of communications for NHS England, tweets...

A spokesman for the Oxford University Hospitals Trust said they have been notified about the cyber attacks and are "on alert".

"So far, our strong security measures are holding solid. We continue to closely monitor the situation and have asked staff to be vigilant. Protecting patient information is our top priority. 

"We understand that NHS Digital has set up a dedicated incident line, and suppliers are working to find solutions to this ransomware outbreak." 

NHS trusts in Herefordshire and Worcestershire have not been affected by the attack, the NHS Clinical Support Unit has confirmed.

It said doctors' surgeries should continue to run as normal but advises all areas of the NHS to proceed with caution. 

More from that NHS statement:

"A number of NHS organisations have reported to NHS Digital that they have been affected by a ransomware attack which is affecting a number of different organisations.

"The investigation is at an early stage but we believe the malware variant is Wanna Decryptor.

"At this stage we do not have any evidence that patient data has been accessed. 

"We will continue to work with affected organisations to confirm this.

"NHS Digital is working closely with the National Cyber Security Centre, the Department of Health and NHS England to support affected organisations and to recommend appropriate mitigations.

"This attack was not specifically targeted at the NHS and is affecting organisations from across a range of sectors.

"Our focus is on supporting organisations to manage the incident swiftly and decisively, but we will continue to communicate with NHS colleagues and will share more information as it becomes available."

Chris Baraniuk
Technology reporter

Software that locks a computer and demands payment before allowing access again - ransomware - is one of the world's biggest growing cyber threats. 

It certainly looks like that is what has hit the NHS in this case - and one IT firm says eleven of its NHS customers, external have been affected. 

Screenshots shared online, external, purportedly from NHS staff, show a program demanding $300 (£230) in Bitcoin that looks similar to ransomware known as WannaCryptor or WCry, external. 

There's no indication of who is behind the attack yet, nor do we know exactly how it infected NHS systems. But hospitals have been targeted with similar software before - it struck three US hospitals last year.

NHS Digital has just issued a statement saying that 16 organisations from across the NHS have reported that their IT systems have been affected.

Staff have told the BBC they saw this image appear on their screens when issues began happening.