Brittany Ferries' 41,000-tonne flagship, the Pont-Aven, has returned to service after undergoing repairs to its cooling system.
The Pont-Aven travelled to Brest in France to undergo repairs
The £100m ferry has been out of action for nearly two weeks after a cracked cooling valve was discovered.
The valve problem caused 1,200 tonnes of seawater to flood the engine room.
Following repairs in Brest, France, the ship set sail for Santander, Spain, to pick up passengers for a scheduled service to Plymouth.
Brittany Ferries was been forced to cancel several crossings after the fault was found, with hundreds of holidaymakers being re-directed to other ports.
Tuesday's 1400 BST from Plymouth to Roscoff, France, has been cancelled, and crossings will take longer than usual because the ferry is not running at full speed.
A Brittany Ferries spokesman said: "She is not travelling at her full cruising speed because we do not want to push it too hard.
"It is getting back to normal, but it depends on how the generators perform in the course of the next couple of days' operations."
Wednesday's service from Plymouth to Santander will leave at 0800 BST instead of 1300 BST, as scheduled, and the sailing is expected to take 24 hours instead of 18.
The Pont-Aven, which is capable of carrying 2,400 passengers and 650 cars, has only been in service for five months.