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A lifeboat exercise on the world’s largest ship has turned fatal overnight, in what could be considered the most dangerous part of working life on a cruise liner.

Crew onboard Harmony of the Seas, docked at Marseille in France for a stopover on its eight-day Mediterranean cruise, were undergoing training and rescue exercises when a lifeboat, with five people in it, detached from the ship’s fifth deck and plunged 10 metres to the water.

One crew member, reportedly a 42-year-old Filipino man, was killed.

The other four crewmen aboard the lifeboat were also injured, with two of them in critical condition and the other two facing “serious multiple injuries”, according to the deputy mayor of Marseille, Julien Ruas.

The incident as well as the man’s death was confirmed on the Twitter account of Royal Caribbean International, which posted that “we’re keeping our colleagues and their families in our thoughts and prayers”.

The industry body Cruise Line International Association (CLIA) announced the rule several years ago that raising or lowering lifeboats with crew aboard was strictly forbidden, but past incidents demonstrate that the rule is not always followed by cruise lines and their crew. For example, Norwegian Breakaway, a similarly high-profile resort-style ship and the largest in the Norwegian Cruise Line fleet, had a similar incident in July this year, when a lifeboat with four crew members aboard fell from the ship, resulting in two deaths.

Back in 2013, five crew members were killed when aboard a lifeboat during a drill on the Thomson Majesty; the lifeboat dropped 20 metres to land upside down in the water below.

Well-known expedition mariner, Captain Ben Lyons, has in the past been quoted as saying these kinds of drills, involving lifeboats, are widely seen by maritime crews as the most dangerous part of life at sea. “There is a strong sentiment amongst many seafarers that lifeboats (through drills) have killed and injured many more people than they have saved,” he has said.

At 120,000 tonnes and 16 decks, Harmony of the Seas is the world’s largest cruise liner; it is higher than a 25-storey building and five times the volume of the Titanic. It can carry almost 6,400 passengers along with 2,400 crew, and made her maiden voyage only this May.

Investigations into the incident by both local and international authorities have begun.