A major incident has been declared after NHS services across England and Scotland were hit by a cyber-attack.
The prime minister said the incident was part of a wider attack affecting organisations around the world.
Staff cannot access patient data, which has been scrambled by ransomware. There is no evidence patient data has been compromised, NHS Digital has said.
The BBC understands up to 25 NHS organisations and some GP practices have been affected.
Ambulances have been diverted and there has been disruption at some GP surgeries as a result of the attack.
NHS England said patients in an emergency should go to A&E or access emergency services as they normally would.
Dr Anne Rainsberry, NHS incident director, added: "More widely, we ask people to use the NHS wisely while we deal with this major incident, which is still ongoing."
Prime Minister Theresa May is being kept informed of the situation, while Health Secretary Jeremy Hunt is being briefed by the National Cyber Security Centre.
Patient safety
NHS Digital said the ransomware attack was not "specifically targeted at the NHS" and was affecting other organisations.
A massive ransomware campaign appears to have attacked a number of organisations around the world.
Screenshots of a well known program that locks computers and demands a payment in Bitcoin have been shared online by those claiming to be affected.
