P&O Cruises owner Carnival has ordered a new 141,000 tonne cruise liner which it says will be the largest built specifically for the British market.
The liner, costing 559m euros ($807m; £489m) and scheduled to enter service in March 2015, will be built by Italian shipbuilder Fincantieri.
The as-yet unnamed ship will be able to carry a maximum of 4,372 passengers.
Rival Royal Caribbean claims to have the world's biggest cruise ship, Allure of the Seas, weighing 225,282 tonnes.
The 16-deck Allure can carry 5,400 guests and went into service late last year.
Carnival said its new ship, which will sail out of Southampton, would "offer an innovative new design and unprecedented number of passenger facilities".
The company's chief executive Micky Arison said: "The UK continues to be a key growth market for Carnival and innovative vessels such as this new P&O Cruises ship - targeted specifically to British clientele - underscore our commitment to this important cruise region".
Miami-based Carnival operates almost 100 ships and currently has eight ships on order, including the new P&O liner.
Fincantieri boss Giusseppe Bono said the deal demonstrated the company's efforts to secure orders in what is a "particularly depressed market".