P&O Cruises and Cunard have extended the suspension of sailings by a month.
The cruise lines were previously due to resume operations on April 11, but they have been cancelled up to and including May 15.
Customers with a holiday booked up to and including that date will automatically receive credit for a future sailing worth 125% of the cost of their original trip.
The firms are both part of the Carnival group.
P&O Cruises president Paul Ludlow said: “It is clear that our original date of April 11 to resume sailings, which would have been a 30-day pause in operations, is just not feasible in light of the Government announcements last week.
“Regrettably, therefore, we are now extending this pause until May 15.
“Given the current guidelines, it is prudent and practical to extend the pause until normal operations can be resumed.
“As we work through the evolving restriction on ports around the world, future itineraries may be subject to change and so we are looking at how we phase our ships back into service.”
P&O Cruises also announced that Covid-19 restrictions have disrupted the final outfitting on its new ship Iona in Germany, which is expected to delay her introduction to service.
Cunard president Simon Palethorpe said: “Everyone in the Cunard team is aware of the need to support the management and containment of Covid-19 globally.
“This includes protecting the health and safety of our guests and crew. It is therefore right we extend the pause in operations.
“It is too soon to know exactly how this situation will evolve. But I do know that we, the Cunard team, will commit all our resources, energy and talent to weather this storm and come out stronger than ever for our guests.
“We have been sailing for 180 years and we look forward to many more. We will get through these tough times together and look forward to welcoming our guests back on board again soon, when the time is right.”