It comes as a university email seen by The News said it suffered a ransomware attack.
Now the university has confirmed for the first time that IT disruption since April 9 is down to a ‘cyber incident’.
Deputy vice-chancellor Bernie Topham told staff the campus closure due to end on Monday will now be extended to Wednesday.
She said: 'Unfortunately the university has been impacted by a cyber incident.
'This has affected some of our IT systems, which remain offline whilst we work with expert support to investigate the issue and securely restore them.
'As part of this, we have taken the decision to close down the campus and many of our services in order to best protect our systems and information while the investigation is underway.
‘Our focus is on minimising any impact on staff and students.
‘As such, we have been working with an experienced cyber security forensic firm to contain the incident as quickly as we can and begin the process of investigating its cause.
Ms Topham said it was a ‘complex task’ but added the university is ‘not aware of any evidence to suggest that data was taken’.
Staff and students were warned against logging into any university Windows devices on its network.
Regulatory authorities and police have been informed.
A Hampshire police spokesman said: ‘We can confirm that we have received a report relating to a cyber incident at the University of Portsmouth.
‘We will be working with the university to investigate this incident and establish the full circumstances.’