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The ship, which weighs 228,000 tonnes, is at 362 metres (1,188 feet) long just 20 metres shy of the length of the Empire State Building.
It is marginally bigger than its sister ship, the Harmony of the Seas, which STX France delivered to Royal Caribbean in 2016.
It holds an ice rink, water park and full-size basketball court along with 2,700 cabins.
“We said thank you in a fairly tangible way. We actually said it a billion times and each one was one euro,” joked Royal Caribbean boss Richard Fain, referring to the ship’s pricetag.
“The Symphony of the Seas is the biggest liner in the world by less than one percent,” STX boss Laurent Castaing told a handover ceremony in the western French port of Saint-Nazaire.
“That one percent extra shows that together we can always make our ships better.”
Built over two years, the Symphony can hold up to 8,000 people including 2,200 staff and is billed as faster and quieter than the Harmony.
The Symphony will start its maiden voyage on Saturday morning, heading to Malaga in Spain and spending its first season in the Mediterranean before heading to its home port of Miami.
The Symphony is the 13th ship that STX has built for Royal Caribbean and comes as the French manufacturer returns to good times, with orders for two ships a year until 2020 — a rate it has not enjoyed for 15 years.
STX, which has a staff of 8,000 and also builds warships, is currently also building the Celebrity Edge, the first of a series of smaller and more luxurious liners for Royal Caribbean.
In February, Italian company Fincantieri announced it had reached an agreement to buy 50 percent of FTX, after a bitter row between Paris and Rome over the shipyard last year.
French President Emmanuel Macron temporarily nationalised STX last July despite a sales deal his predecessor Francois Hollande had struck with Italy.
The French government said it was ready to seek other buyers if Italy refused a 50-50 split — a proposal angrily rebuffed by Rome, but which it has since accepted.
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