The cancellation of Condor Liberation and Rapide passenger sailings have been extended until 21 May.
It follows government advice about travel restrictions.
The lockdown has been extended until 11 May.
The cancellations were originally set until 15 May because of coronavirus but have been extended again for another week.
"In the interest of public health we have had to react to these restrictions; we are sorry for the change to your trip.
"Everyone affected by the suspension will be offered a rebooking or travel voucher which is valid for 18 months.
"Our contact teams will be extremely busy during this period and ask for your patience due to the unprecedented changes to the schedule." - Condor Ferries.

Waiting times to see a medical specialist in Guernsey still below target
'Small' bomb mistakenly taken into Guernsey Police Station
'Useful' amount of rain expected in the Channel Islands
Four rescued from RIB 40 miles west of Guernsey
Stark warning over Guernsey's changing climate
Possible job cuts at RBSI in Guernsey and Jersey
Guernsey students beat England's A level pass rate
Guernsey community unites out west for solar eclipse