The cruise lines had previously cancelled all trips up to May 15.
Customers with affected bookings will automatically receive credit for a future sailing worth 125 per cent of the cost of their original trip.
Statements issued by the firms, which are both part of the Carnival group, did not mention if cash refunds would be offered.
The announcement means the launch of Iona, the latest ship to be operated by P&O Cruises, will be delayed.
P&O Cruises president Paul Ludlow said: “With the current lockdown in place and government guidelines around the world advising against travel, it is necessary to extend the pause in our operations for all sailings up to and including July 31, 2020.
“We are so sorry not to be able to give our guests amazing holidays at the moment, but we will do so as soon as global guidelines will allow.”
Sailings for Cunard's Queen Elizabeth have been cancelled up to September 8.
Cunard president Simon Palethorpe said: “These are extraordinary times and this global outbreak continues to challenge Cunard and the travel industry as a whole.
“We would love nothing more than to bring our fleet back in to service, and give our guests the holidays they deserve and long for but, with the lockdowns and travel restrictions in place around the world, this is simply not possible at this time.”