P&O staff are being told by the RMT union it will resist any compulsory redundancies, following the firm's decision to cut 1,200 jobs.
P&O is cutting routes to save costs
Four of P&O's 13 ferry routes and eight ships from its fleet are to be axed.
Workers at Dover, Portsmouth and Hull are to hold a series of meetings to discuss the cuts announced on Tuesday.
The RMT has criticised the way the company made its announcement, which follows increasing competition from no-frills airlines and Eurotunnel.
Most job losses are expected at Portsmouth and Dover, but BBC Radio Humberside has also learned staff on the ship Pride of Hull will have to apply for a reduced number of vacancies.
RMT Union general secretary Bob Crow said: "Our members will be devastated by these massive cuts.
"We urged the company to consult with us properly, but instead they kept us guessing for weeks and then gave us a massive pile of documents after they had made their announcement.
"That is not consultation and loyal P&O workers deserve better than this knee-jerk reaction."