Wheelchair users won't be able to travel on Wightlink's FastCat service until June at the earliest, after the £1.5m upgrade of the firm's Portsmouth terminal suffered a setback.
It was hoped the second phase of the project, which started on January 5, would be completed within three months.
However, Wightlink says work on the 100-year-old terminal is taking "longer than expected".
A spokesperson for the firm said:
"Wightlink’s £1.5million investment in its Portsmouth Harbour Fast Cat terminal is taking longer than expected because of its decision to carry out more work on the building, which is more than 100 years old.
"This includes completely renewing the roof of the passenger waiting area as the original structure was beyond repair."
At the time, the company said no prams, wheelchairs or mobility scooters would be able to access the service because there are stairs on the alternative outdoor route.
This remains the case, and customers are advised to travel by car ferry from Gunwharf Road instead.
Wightlink says it aims to have the terminal back up and running in time for the Isle of Wight Festival (16-19 June).
"The company is confident that the passenger ramp, and reduced mobility access will be restored in June, in time for the Isle of Wight Festival. The terminal building is set to be fully reopened in time for the peak holiday season.
"Wightlink thanks its customers for their patience during these essential works which will secure the future of the terminal."
A temporary terminal has been erected in Portsmouth train station since works began.