The council claimed the boat pulled out a length of timber
A restored 1940s paddle steamer has damaged part of Worthing Pier while trying to dock, the council has said.
The vessel, which was heading for the Isle of Wight, hit the end of the pier on Monday, according to the authority.
It said part of the structure is closed but Pier Day - an afternoon of tours and stalls - will go ahead on Sunday.
Waverley Excursions, which operates the vessel, said there was a "local failure" on part of the pier and the landing stage "became dislodged".
A spokeswoman for the operator said the boat had sailed to Worthing to pick up passengers for a cruise.
The ship had to depart quickly before the passengers could be embarked
She said: "After berthing, with all lines securing the ship to the bollards on the pier, Waverley's crew were making the gangway ready to board passengers.
"A local failure on part of the structure of the landing stage meant that part of the landing stage became dislodged, and the ship had to depart quickly before the passengers could be embarked.
"We understand that the main pier was completely unaffected."
She said the vessel was unaffected by the incident.
But Worthing Council said it was found the vessel had pulled out a 14.7ft (4.5m) length of timber, leaving a section of the iron gratings unsupported.
Specialist timber needed to mend the pier would take at least three weeks to arrive and repairs would take place during "a decent low tide and good weather".
The council said: "The pier remains open, with only a small part of the structure where the damage occurred closed."
The south section of the landing stage will be closed for about eight weeks, and the east and west sides remain open, it added.
The council is liaising with the boat's insurers, but no details were available about the cost of the damage, it said.