University students across the UK have been unable to submit work, after the academic computer network known as Janet came under cyber-attack.
Distributed denial of service (DDoS) attacks began on Monday and are continuing, according to the network's operator, Jisc.
The attacks "have resulted in reduced connectivity and disruption", says a statement on Jisc's facebook page.
Engineers are working to restore normal service, it adds.
DDoS attacks are malicious attempts to interrupt or degrade an internet-connected service, often by flooding that service with large amounts of network traffic.
Jisc, formerly the Joint Information Systems Committee, is a publicly funded body that provides computer network services to higher education and research bodies.
Its statement says it is using "various blocks and filtering" to limit the impact of the disruption, "but the details of each attack are subject to change".
The body says it has had to limit its tweets "as we suspect that those behind today's DDoS attack are adjusting their point of attack based on our Twitter updates".
It also had to close its own website for a period, as it also came under attack.
"We understand the importance of connectivity to colleges, universities and other public sector organisations," said Jisc executive director Tim Kidd.
"We are doing everything in our power to ensure normal service in resumed as soon as possible, and in the meantime to minimise any disruption that users of the Janet network may be experiencing. We apologise for any inconvenience caused."
University of Manchester, one of the universities affected by the attack, said staff and students had experienced intermittent problems accessing external sites.
Emails to staff and students warned them to expect "slow performance or a complete lack of access to external services".
"By flooding the service with excessive network traffic, an attacker is attempting to exceed the capacity of the service, which causes the service to run slowly or become unavailable," the university said.
It added staff and students were experiencing problems with accessing external websites, email, submitting assessments online and external collaboration services for research staff.