Commodore Clipper's sailings today and tomorrow have been cancelled because of a problem with her port main engine.
She is alongside in St Peter Port and engineers hoped that a repair they carried out today would have fixed the problem.
But that has been unsuccessful. They will be working through the night to try to fix it.
Condor said it anticipates Clipper will will sail from Guernsey to Jersey at approximately 10 tomorrow and resume normal service from Portsmouth on Wednesday at 9am.
Final confirmation regarding revised times for tomorrow will be issued by 7am.
The fast ferry, Condor Rapide, was running on reduced speed today due to a fuel pipe problem and is not expected to be back up to full speed until tomorrow afternoon.
Condor has confirmed the Clipper's engine has been repaired and it will leave for Jersey and Portsmouth at approximately 10am