(Bloomberg) — Carnival Corp., the world’s largest cruise operator, said it will buy a $739.9 million vessel to expand its flagship cruise line.The company exercised an option to build the 130,000-ton, 3,652-bed ship for its Carnival Cruise Lines brand.

Fincantieri- Cantieri Navali Italiani SpA will construct the vessel, which is scheduled for delivery in 2010, Miami-based Carnival said today in a statement.

Carnival Cruise Lines is the biggest of the company’s 12 brands, comprising 21 of its 81 ships. The vessel that was announced today is a sister ship to one scheduled to be finished in 2009.

The ships will form a new class for Carnival Cruise Lines. They will be the company’s largest in terms of bed number, said Carnival spokesman Vance Gulliksen. Carnival Cruise Lines now has four new ships on order or under construction with Fincantieri, Italy’s state-owned shipbuilder.

Carnival overall has 19 new ships on order to be finished between March 2007 and 2010. The new orders will increase the company’s fleet by 23 percent.