Tui Cruises has ordered two 2,500-passenger ships for delivery in 2016 and 2017.
The move will double Tui Cruises’ fleet to six ships offering 14,000 beds by 2017.
The order by the joint venture between German-based Tui AG and Royal Caribbean Cruises comes as Germany is set to overtake the UK as Europe’s largest cruise market.
Tui AG chief executive Friedrich Joussen said: “In Germany and in Europe the interest in cruises is increasing.
“The Mein Schiff fleet of Tui Cruises has set a new premium standard in the market and it won new target groups for cruise holidays.
“The ships are excellently booked to capacity and the company is growing.
“Tui Cruises and its vessels are a pivotal pillar for our differentiation and growth strategy. Cruises meet the trend and are being discovered by new customer segments.”
Tui AG is merging with UK-based Tui Travel, Europe’s largest tour operator, in which it has a majority stake.
The new vessels will be sister ships of previous new-builds Mein Schiff 3 and Mein Schiff 4 with 15 decks and 1,250 cabins. Ninety per cent of the cabins will have ocean vies with 82% having balconies.
Both new 97,000 tonne ships will be built at STX shipyard in Turku, Finland.
The line’s first new-build, Mein Schiff 3, entered service in June and Mein Schiff 4 is under construction.