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 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".
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.
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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.
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