P&O is to axe four of its 13 ferry routes with the loss of 1,200 jobs and the transfer of a further 350 posts.
It is cutting its ferry fleet from 31 to 23 ships and dropping all its Western Channel routes except Portsmouth-Bilbao.
Tourist traffic had been "adversely affected by fewer day trips and the expansion of the low-cost airlines," said a spokesman.
P&O is axing its Portsmouth-Cherbourg, Portsmouth-Caen, Portsmouth-Le Havre and Rosslare-Cherbourg routes.
Brittany Ferries will take over its Portsmouth-Le Havre route, chartering two of its vessels.
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