A number of procedures have been cancelled or redirected to other NHS providers after a cyber attack affected major hospitals in the capital.
King's College Hospital, Guy's and St Thomas', including the Royal Brompton and the Evelina London Children's Hospital, and primary care services were hit by the "major IT incident" involving pathology partner Synnovis, letters sent to NHS staff said.
Trusts reported the incident was having a "major impact" on the delivery of services, with blood transfusions particularly affected.
A spokesman for King's College Hospital in London confirmed it was affected.
The cyber incident is thought to have occurred on Monday, meaning some departments could not connect to their main server.
Several senior sources have told the Health Service Journal (HSJ) the system has been the victim of a ransomware attack.
One said gaining access to pathology results could take "weeks, not days".
"Non-essential transfusions were paused when the incident emerged last night," said Sky News's science and technology editor Tom Clarke.
"Anyone needing emergency transfusions, they're having to proceed with pen and paper as I understand it. Obviously, that will slow down the matching of blood and things like that."
Read more from Sky News:
Male birth control applied as gel works faster than other methods
AI to watch every spoonful you eat
England's first artificial cornea transplant hailed a success
There are suggestions urgent and emergency care at the hospitals will be affected as they may not be able to access quick-turnaround blood test results.
Synnovis, the partner involved in the major incident, was formed from a partnership between SYNLAB UK & Ireland, Guy's and St Thomas' NHS Foundation Trust, and King's College Hospital NHS Foundation Trust.
Synnovis describes itself as "one of the largest pathology service providers in the UK" and aims to improve patient outcomes through "faster testing services and state-of-the-art laboratories", according its website.
There have been a number of recent attacks of NHS groups.
Last month, a ransomware group released stolen patient data onto the dark web after an attack on NHS Dumfries and Galloway in March.
Keep up with all the latest news from the UK and around the world by following Sky News
Tap hereJulie White, chief executive of NHS Dumfries and Galloway, branded it an "utterly abhorrent criminal act".
"We should not be surprised at this outcome, as this is in line with the way these criminal groups operate," she said.
The health board is urging the public to be alert for any attempts to access their work and personal data.
Disclaimer: The copyright of this article belongs to the original author. Reposting this article is solely for the purpose of information dissemination and does not constitute any investment advice. If there is any infringement, please contact us immediately. We will make corrections or deletions as necessary. Thank you.
