London, May 13: A cyber attack which hit Britain's National Health Service (NHS) was declared as a major incident Friday night, with British Prime Minister Theresa May saying it was an international attack.

Hospitals, health clinics, family doctor surgeries across England and Scotland were seriously affected by what was the biggest ever attack on the NHS, Xinhua reported.

Hospital emergency rooms said only the most serious cases could be handled, surgical operations had to be cancelled, and thousands of health staff were unable to access vital patient information. Some hospitals were forced to divert patients to alternative hospitals for treatment.

May told a media briefing: "We are aware that a number of NHS organisations have reported that they have suffered from a ransomware attack. This is not targeted at the NHS, it's an international attack and a number of countries and organisations have been affected.

"The National Cyber Security Centre is working closely with NHS digital to ensure that they support the organisations concerned and that they protect patient safety. We are not aware of any evidence that patient data has been compromised," she added.

With at least 25 hospitals and health trusts hit in the attack, NHS Digital said there was no evidence patient data has been compromised.

Anne Rainsberry, NHS incident director, said: "We are asking people to use the NHS wisely while we deal with this major incident, which is still ongoing."

She said there are contingency plans to keep the NHS open for business.

Politicians campaigning for next month's general election have seized on the attack to say underspending on health services have left the NHS vulnerable to cyber crime.

Cyber attack disrupting British NHS was international: May