Hospitals affected are believed to have lost the use of phone lines and computers, with some diverting all but emergency patients elsewhere.
It had to cancel routine appointments and ambulances were being diverted to neighboring hospitals, Barts said.
The NHS Digital denied reports that it was a targeted attack only on NHS hospitals and said other organisation across a range of sectors including Global Positioning System are also affected.
She also said the ransomware hit was "not targeted" at the health service but was part of a wider assault on organisations across a number of countries, and added that the National Cyber Security Centre (NCSC) is working to support the NHS. The attack reportedly shuts down its targets and then asks for $300 worth of ransomware.
"Both staff and patients were frankly pretty appalled that somebody, whoever they are, for commercial gain or otherwise, would attack a health care organization", he said. The committee denied the reports.
Meanwhile, Russian media also reported cyberattacks on the Interior Ministry.
Outlets are reporting that more than a dozen countries - and as many as 74, according to one security firm - have been affected by the hacking tool. "We are implementing remediation steps as quickly as possible".
The cyber-attacks appear to have have affected several organisations worldwide.
Forcepoint Security Labs said that "a major malicious email campaign" consisting of almost five million emails per hour was spreading the new ransomware.
In Spain, the attacks did not disrupt the provision of services or network operations, the government said in a statement.
Similar ransomware attacks were reported in Italy and Ukraine.
A ransomware virus is reported to be spreading aggressively around the globe, with over 50,000 computers having been targeted.
Hospitals in the United Kingdom were crippled by a "large-scale" cyber attack on Friday.
Spain's Telefonica was among the companies hit.
"We have around 70 computers that have had a unsafe code installed", said Andreaz Stromgren, the mayor of Timra, about 400 km north of the capital Stockholm. It's an encryption-based ransomware that locks a computer unless money is paid to gain access. Attacks of this kind have spiked in the previous year, jumping from 340,665 attacks in 2015 to 463,841 in 2016, according to Symantec.
"Today our engineers added detection and protection against new malicious software known as Ransom:Win32.WannaCrypt", a Microsoft spokesman said in a statement.
"I was actually in my bed most of the day, they prefer you not to get out of bed".
Security experts are urging organizations to patch vulnerable systems, upgrading to the latest versions of OSes, and making backups of any critical files. If a payment is not made within a week, files will be permanently deleted, the threat continued.
- Derby hero Griezmann won't rule out Real Madrid move
- The history of Mother's Day
- Oakland Raiders take UConn's Obi Melinfonwu in second round of NFL Draft
- Packers draft CB King, S Jones, DT Adams
- Speculation: The LA Clippers should consider Kyle Lowry if Chris Paul leaves
- 'Guardians of the Galaxy Vol. 2' rockets to No. 1
- PM, chief ministers travel to China to attend Belt and Road Forum
- Celtics' Isaiah Thomas Reveals His Top 5 Candidates For NBA MVP
- Oil prices rise in Asia in expectation of Aramco supply cut
- Protesters call for investigation following FBI director firing