NHS bosses and the government are facing questions over why hospitals had been left vulnerable to the global cyber attack that crippled services on Friday.

The health service faces a weekend of chaos after hackers demanding a ransom infiltrated the health service’s antiquated computer system.

Operations and appointments were cancelled and ambulances diverted as up to 40 hospital trusts became infected by a “ransomware” attack demanding payment to regain access to vital medical records.

Doctors warned that the infiltration – the largest cyber attack in NHS history – could cost lives.

Medics described how computer screens were “wiped out one by one” by the attack, which spread to companies and institutions worldwide, including international shipper FedEx Corp in the US, and Germany's rail operator.

'Biggest ransomware attack in history'

Researchers with security software maker Avast said they had observed 57,000 infections in 99 countries with Russia, Ukraine and Taiwan the top targets.

Mikko Hypponen, chief research officer at the Helsinki-based cybersecurity company F-Secure, called the attack "the biggest ransomware outbreak in history".

The NHS said there was no evidence that patients’ medical records had been accessed, but it was unable to say whether the hackers – who are threatening to delete information unless payment is received within a week – had the ability to destroy such records.