Workers across the Republic of Ireland and Northern Ireland have been placed on high alert over cyber security.
Family doctors in Ireland have been warned not to use their practice computers on Monday until they are certain their anti-virus security systems are up to date after a mass hack which has spread around the world since Friday.
The PSNI said some people's systems may already be infecting and waiting to activate after the weekend.
More than 200,000 computer systems in nearly 100 countries have been infected with the so-called WannaCry ransomware since Friday.
The software locks files or a whole machine until a ransom is paid.
The malware has caused severe disruption to health services, industry, transport, academia and law enforcement around the world.
Detective Chief Inspector Douglas Grant, head of PSNI Cyber Crime Centre, said: "While no incidents have been reported in Northern Ireland as yet, we do expect further infections to come to light globally over the next week and we cannot rule out the possibility that local systems may be vulnerable.
"As people return to work tomorrow after the weekend, many will have unopened, potentially infected emails in their inboxes, or their systems may already be infected and are waiting to activate.
"It is of the utmost importance that individuals and organisations act to ensure the integrity of our local cyber networks and take appropriate action to reduce the threat posed by cyber criminals."
In the Republic, the Health Service Executive (HSE) cut its network off from external communications as a precaution.
The decision means its facilities will be unable to receive emails from outside the organisation.
But HSE chief information officer Richard Corbridge said the impact on patients is unlikely to be significant because email is rarely used to communicate with patients.
The PSNI said the latest advice and patches are now available to protect current and older legacy systems via: https://www.ncsc.gov.uk/guidance/ransomware-latest-ncsc-guidance