It applies to MSC Virtuosa’s sailings around the British Isles and P&O’s UK coastal voyages onboard Britannia and Iona departing between 8 July and 24 September.
MSC do not require fully-vaccinated travellers to take a Covid test, while all P&O guests - including those with both jabs - will need to provide evidence of a negative PCR test.
On Tuesday (29 June), MSC announced an amendment to its policy, requiring adult guests to be fully vaccinated owing to what it described as a "change in government policy" to allow higher passenger capacity.
According to the DfT, current restrictions - allowing cruises to operate with up to 1,000 people or 50% capacity, whichever is lower - still remain.
A spokesperson for P&O said: "This vaccination and testing policy will be reviewed on a regular basis as the global situation continues to evolve.
"At the time of travel, some ports of call may stipulate specific vaccine policies for guests going ashore that differ from this policy."
