The ferry company, Red Funnel, says it believes a "malicious attack" on its systems may be preventing customers from making a booking.
Since Monday, computer problems have stopped passengers buying tickets for the Southampton to Isle of Wight route.
Customers are currently unable to use the following services:
Online and telephone bookings
Customer account enquiries
Automatic NPR (number plate recognition) at check-in
Live service status updates
Our current assessment of the problem suggests that this is a malicious attack designed to strain our operation and bring our systems to a halt. Data theft does not appear to be a motive.
The company says several of its internal systems have been corrupted, including its booking and timetable management systems.
Fran Collins, CEO at Red Funnel, said there is no evidence to suggest any personal data leaks and she wants to reassure customers that credit and debit card details are not held within their systems.
The company says it is working hard to resolve the situation and it is anticipated that a basic version of its online booking system will be resumed within the next 24 hours.
Red Funnel rewards points and travel cards are currently not being accepted on new bookings.
Passengers who are travelling with existing bookings are asked to arrive in advance of their sailing time to ensure sufficient time to process payments.
Since Monday, all passengers who have turned up to travel without a booking have been able to get on their desired sailing.
The company says it wants to thank customers and staff for their patience during this time.