(5:45 p.m. BST) -- The Department for Transport has ruled that cruise ships sailing with all adults fully vaccinated can sail half full from July 19.
Previously lines had to sail half full or at a 1,000 passenger limit, whichever is less, meaning big ships like MSC Virtuosa and Anthem of the Seas have been forced to sail a sixth or a quarter full, respectively.
The rule does not apply to under-18s, so children can continue to sail on those ships not mandating all passengers be fully vaccinated.
CLIA UK and Ireland managing director Andy Harmer, said: "We welcome the news that as of 19 July, if the next step of the government roadmap goes ahead, the 1,000-passenger limit on domestic cruises can be lifted to enable larger ships to sail at 50% capacity.
"As the cruise industry’s phased return picks up, this is another positive development. We continue to work with the government on the regulations for domestic and international cruising, with the view to ultimately achieving a full return to service."
Cruise lines have been anticipating this move, which follows the announcement today that double-vaxxed Brits will be able to fly back from "amber list" countries without having to quarantine.
MSC Cruises changed its vaccination policy just a few days ago from a mixed approach -- either double vaccinated or a negative PCR test -- to one mandating that all adult passengers must be fully vaccinated in order to increase its passenger number from 1,000 to more than 3,000.
And yesterday, Jo Rzymowska, Celebrity's vice president and managing director for Europe, Middle East and Africa, told Cruise Critic that the line was ready to increase passenger numbers on Celebrity Silhouette to around 1,500 as soon as government guidelines allowed:
"The capacity limit is being lifted imminently to 50 percent of capacity, and we are ready to go with that," Rzymowska said from onboard Silhouette's first sailing in 15 months.
"The first three sailings are sold out at the capacity that we are at, so we will be able to open it up and get more people onboard, and that's what we are looking to do."