Specifically pointing to USA intelligence agencies, Smith said the worldwide hack "provides yet another example of why the stockpiling of vulnerabilities by governments is such a problem".
Basildon Hospital was not affected by the malware, but as a precaution has taken steps to protect its internal IT systems. Bossert acknowledged that "attribution is difficult" at this point. "Machines were shutting down but no clinical information was affected", he said. He said infection rates slowed over the weekend.
"Our security protection methods have prevented significant impact of this attack within our organisation".
The initial attack, known as "WannaCry", paralyzed computers that run Britain's hospital network, Germany's national railway and scores of other companies and government agencies worldwide in what was believed to be the biggest online extortion scheme so far.
Seven hospital trusts are still experiencing serious problems, among them St Bartholomew's Hospital in London, York Teaching Hospital NHS Trust and the University Hospitals of North Midlands Trust.
The malware, using a technique purportedly stolen from the US National Security Agency, stopped care Friday at hospitals across the United Kingdom, affected Russia's Ministry of Interior and infected company computer systems in countries from Eastern Europe to the US and Asia.More news: Prince William pays tribute to late mother Diana's legacy
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An investigation is under way to identify the cause of the attack and ministers are to convene an extraordinary meeting of the National Cyber Resilience leaders' board on Tuesday to review the response to the breach.
Highlighting an incident at Papworth Hospital near Cambridge where a nurse clicked on a malicious link and malware infected her computer and started to encrypt sensitive files, he wrote in his study, "Fortunately, the hospital's daily data backup had just been completed".
Prevention is the best way to avoid becoming a ransomware victim.
The organisation also recommended that trusts ensure security software patches were up-to-date and that up-to-date virus software was also being used.
Hackers can still gain easy access to personal computers that lack a security update issued in March by Microsoft to fix the vulnerability in its Windows operating system.