A spokesman for the Royal Marsden NHS Foundation Trust (RMFT) said on Friday afternoon that it was experiencing a “major network issue” .
He confirmed the problem was related to the cyber-attack which may have affected up to 40 NHS trusts in England.
London North West Healthcare NHS Trust confirmed it has not been affected by the attack so far, although it is "closely monitoring" its IT systems.
Imperial College NHS Healthcare Trust is also unaffected.
Barts Health NHS Trust, which runs five hospitals in the City of London and east London, said it was experiencing “major IT disruption”.
A spokesman for the trust said: “We have activated our major incident plan to make sure we can maintain the safety and welfare of patients."
The National Cyber Security Centre said it was working with NHS Digital and the National Crime Agency to investigate the attack.
One theory is that it involves “ransomware”, a bug that blocks devices until the user agrees to pay a fee.
A statement released by the NHS on Friday evening acknowledged attacks on at least 16 NHS trusts.
It stressed an investigation was at an early stage but it was believed the attack involved malware called Wanna Decryptor.
"At this stage we do not have any evidence that patient data has been accessed," a spokesman said.
"We will continue to work with affected organisations to confirm this."
Wanna Decryptor first appeared in February this year.
It works by encrypting files on target computers before demanding a ransom be paid in web currency Bitcoins.
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