Around 100 passengers are currently stranded on board the Condor Clipper off Portsmouth.
Its after what is thought to be a WW2 bomb was discovered in the harbour's waters.
A Navy bomb disposal unit is on the scene and there is 500 metre exclusion zone around the suspected device.
The harbour will now be closed all day, leaving the Clipper stuck after leaving Jersey last night.
In a statement the ferry operator said:
Due to a possible unexploded ordinance found in Portsmouth Harbour, the harbour is currently closed.
Commodore Clipper is currently holding outside the harbour and we will send out an update once further information is received.
The Navy say this isn't the first time a device has been found.
Another German UXB has been found in Portsmouth harbour.
A 500-metre cordon is in place around the device which is preventing cross-channel ferries from entering and leaving the ferry port.
A plan is in place to tow the bomb out of the harbour where it will be destroyed in open waters by RN bomb disposal experts.
The device was found by a dredging barge carrying out work in the harbour before the arrival of HMS Queen Elizabeth - the navy's new 65,000-tonne aircraft carrier - into the naval base next spring.
Other ferries have been left stranded off the coast as well.
Condor Clipper services have been cancelled today after a WWII bomb was discovered in Portsmouth Harbour.
Condor Ferries has confirmed they have been cleared to enter Portsmouth Harbour, after a WWII bomb was found there this morning.
Condor Clipper remains stranded outside Portsmouth. It's been announced part of the harbour will be closed all day after a bomb discovery.