Union members at P&O have voted in favour of industrial action in protest at plans to cut 1,200 jobs.
The RMT union said ferry workers at Portsmouth and Hull had voted for strike action, while Dover was in favour of action short of a strike.
RMT general secretary Bob Crow said strikes at Portsmouth and Hull could take place over Christmas and New Year.
He said on Tuesday the decision of union members at Dover not to endorse a strike would be respected.
"Our members at Dover, Hull and Portsmouth have voted for industrial action against P&O's slash-and-burn job cuts and against attacks on their pay and conditions.
"There is a clear mandate for strike action at Hull and Portsmouth.... members at Dover, despite bearing the brunt of appalling job losses over the last decade, have voted for industrial action short of strike.
"RMT is a democratic union and their decision... will of course be respected," Mr Crow said.
He added that meetings would be held this week to discuss the campaign of action at all three ports.
P&O announced in September plans to make redundant 800 workers at Dover and 400 at Portsmouth. Some at Hull are also affected.
The RMT balloted more than 1,000 union members who work at P&O.