MIAMI – Royal Caribbean International yesterday ordered the world’s largest and most expensive cruise ship, a $1.24 billion vessel that will hold up to 6,400 passengers, the latest sign of an industry trend of supersizing ships.

The ship, dubbed Project Genesis, will be 220,000 gross registered tons when it is delivered to the world’s second largest cruise operator in fall 2009 by Oslo, Norway-based shipbuilder Aker Yards. Gross registered tons is a standard way to measure a ship’s size and is a unit of volume equal to about 100 cubic feet.

The ship will weigh about 100,000 tons based on displacement – a Nimitz-class aircraft carrier comes in at about 97,000 tons.

Aker said its contract price of 900 million euros, or about $1 billion, would be “the most valuable ship ever ordered in the history of commercial shipbuilding.” The $1.24 billion figure includes all expenses for the ship, “from forks and knives and sheets to artwork and everything else,” the Miami-based cruise line’s executive vice president of maritime operations, Harri Kulovaara, said.

Aker said the contract is contingent on final approval of financing and the ship will be built at one of its Finnish yards. The cruise line, a unit of Royal Caribbean Cruises, has an option for a second ship.

“Project Genesis truly is a remarkable ship. Its bold design, daring innovations, and technological advancements will delight our existing cruisers and help us draw in new ones,” the parent company’s chairman and CEO, Richard Fain, said in a statement.

The announcement also steals some of the spotlight from rival Carnival Corporation, the world’s largest cruise operator. Carnival has studied building a ship about the same size, but its Pinnacle project is “on the back burner” because of its prohibitively high price, a spokesman, Tim Gallagher, said.

Mr. Kulovaara said in a phone interview that the new ship will be more fuel efficient than current vessels, but he declined to give a specific figure. He said plans for the types of onboard amenities were being finalized. Royal Caribbean has been an innovator in featuring ice skating rinks, rock climbing walls, and surfing pools.

Royal Caribbean’s ships are typically more upscale than the bargain Carnival Cruise Lines’ vessels, but they aren’t as traditional as those of luxury carriers such as Cunard.

Project Genesis will carry 5,400 passengers based on two people per cabin, Mr. Kulovaara said. But as most cruise cabins can accommodate more than two people using cots or other beds, that number rises to a maximum capacity of 6,400.

The owner of the discount online travel agency Cruisequick.com, Ray Weiller, said many of his clients are drawn to the ever-growing size and number of amenities of ships, but others tire of waiting in long lines to get on and off the vessels. Carnival’s Cunard Line currently has the world’s largest cruise ship – the Queen Mary 2 – at 151,400 gross registered tons. But Royal Caribbean is scheduled to get an even bigger ship in June, the 160,000-ton Freedom of the Seas. It will carry 3,600 passengers double occupancy and 4,370 maximum.