MSC previously only required guests to show proof of a negative COVID test result, but they will now require all adults to be fully vaccinated for the summer sailings.
MSC said it had made the decision because the government was allowing companies to "increase the number of guests permitted onboard", on the condition adults are fully jabbed.
The company said its original policy had been focused "on inconveniencing as few guests as possible", adding it would be contacting any affected passengers with bookings to discuss options like rebooking / receiving a refund.
The company released the following statement:
"The UK government’s decision to increase the number of guests permitted onboard round-the-UK cruise ships to up to 50 per cent of passenger capacity is to be welcomed as it provides more options for British holidaymakers.
"The condition of this change is that all adult guests must be fully vaccinated and our focus was on inconveniencing as few guests as possible in this rapidly changing environment. It will, however, understandably frustrate a number of our guests who had already booked their holiday with us for voyages but are not yet fully vaccinated.
"While we regret that this change to the capacity limits will disrupt their travel plans and our ability to accommodate some guests as originally planned, the capacity increase allows us to protect many more guests’ holidays which would have been otherwise affected by the recently announced delay of the easing of COVID restrictions.
"We are now contacting affected guests to offer them options for their holiday, including rescheduling, refunds and exemptions."