The ferry was approaching Holyhead when it had a "soft impact" Thousands of passengers have had travel plans disrupted after a ferry struck a whale just off Holyhead in north Wales.
Irish Ferries have confirmed its Dublin Swift service would be in dock for two days following the incident at 1400 BST on Tuesday.
The "soft impact" collision happened as the vessel approached the port of Holyhead. No one was injured. A spokesman for
Irish Ferries said the 4,000 passengers booked on the sailings will be accommodated on the car ferry.
The company said the Jonathan Swift "may have hit a whale or another underwater mammal". Sales manager Declan Mescall said: "We're not too sure exactly what happened. "We certainly hit a submerged object this afternoon, about half an hour out from Holyhead port.
"The Jonathan Swift hit something big and soft and the suggestion is that it's some kind of sea mammal." He said the vessel was travelling at about 40 knots at the time of the impact.
He added it was not unusual for dolphins to play alongside the firm's larger car ferries as they made the crossing.
Irish Ferries said the Jonathan Swift was being taken out of service as a precautionary operational measure.
Over the next two days divers will inspect how much damage, if any, has been caused. A spokesman said: "Depending on the outcome of that investigation, and whether or not repair works have to be carried out, that time scale could be shorter or longer."