French emergency workers have called off a search and rescue operation after unsuccessfully searching for the man through the night.
The man fell from the Celebrity Eclipse, which was off the French coast, at about 2215 BST on Friday.
It is not known if he was a passenger or crew member. His nationality is yet to be confirmed.
Very cold
"Unfortunately there was a man in the sea. We searched for him extensively but we could not find him," a French coastguard official said.
"We deployed our resources to find him but it was to no avail and we can now presume that he is dead.
"The water was very cold and there is no hope for him."
Solent Coastguard Maritime Rescue confirmed it had received information that a person onboard the cruiser had fallen overboard but did not know if he was British.
The Celebrity Eclipse is owned by travel company Celebrity Cruises. It is believed to have been travelling between Cherbourg and Southampton when the incident occurred some eight miles north of the French port.
Celebrity Cruises offer trips around Europe and the Mediterranean, as well as the Antarctic. The Eclipse cruiser, a Solstice class vessel, joined the fleet last year, according to the company's website.
