The largest NHS trust in England has been hit by a cyber-attack that could affect thousands of files across at least four London hospitals.

Barts health trust, which runs five hospitals in east London – the Royal London, St Bartholomew’s, Whipps Cross, Mile End and Newham – has sent a message to staff urging them not to open email attachments from unknown senders.

“We are urgently investigating this matter and have taken a number of drives offline as a precautionary measure, a Barts spokeswoman said. “We have tried and tested contingency plans in place and are making every effort to ensure that patient care will not be affected.”

It was reported earlier on Friday that the trust had been targeted with ransomware, which is normally delivered via emails that trick the recipient into opening attachments and releasing malware on to their system. But the spokeswoman ruled out such an attack on Friday night.

The trust has not said how much of its system has been affected by the attack or whether patient data has been compromised but it said it believed that most of the affected system was housing corporate data.

The trust’s filing system between departments has been turned off while the investigation takes place.

Staff at the Royal Free London foundation trust were also warned to beware of attacks on Friday, the Guardian has learned.

“We have been informed of a major cyber-attack on NHS organisations. Please exercise extreme caution when opening any email attachments from unknown source or that don’t seem relevant to you. We will be carrying out security scans on all computers within the trust so please leave them switched on until further notice,” wrote the trust’s IT director, Tosh Mondal.

A spokesman said the email was in reaction to the Barts attack and that the Royal Free London, as well as Barnet and Chase Farm hospitals, had not been affected.

NHS Digital said it was aware that Barts had been infected by a “virus which has affected their IT systems”.

A spokesperson said: “This issue highlights the fact that there are threats to data security within the health and care sector, as with any other sector. We remain committed to supporting the protection of data with the highest possible security standards, high levels of security expertise from the centre and appropriate training and awareness of the risks for all staff.”

She declined to answer questions about whether other NHS trusts had been affected, how much data may have been affected and who may be behind the attack.

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