Originally called the Free Enterpise IV and one of eight similar vessels operated by Townsend Thoresen, it had been "pensioned off to the Third World".
Another sister ship of the Herald, the Al-Salaam '95, sank in the Red Sea in October after a collision with a Cypriot commercial vessel. A 16ft-wide hole resulted in the ship sinking within three-and-a-half minutes. In that case almost all of the passengers were rescued.
Andrew Linington of Numast, the British ships' officers union, said that, under European regulations introduced after the Herald of Free Enterprise sinking it would be illegal to operate either vessel in Europe. "This is a scandal. Passenger ships like it are not permitted to sail in European waters. If a ship is unsafe, it is unsafe wherever it operates."
The rules introduced in the late 1990s ensured that "roll-on, roll-off" ferries were redesigned. Stability was increased so that an initial ingress of water was not necessarily catastrophic.
Mr Linington said that, in developing countries, an average of 1,000 people a year had lost their lives in ferry sinkings over the past decade, largely because legislation was not as stringent.
"It's a roll-on, roll-off ferry and there is a big question mark over the stability of this kind of ship," said David Osler of the London shipping paper Lloyd's List. "It would only take a bit of water to get on board this ship and it would be all over ... The percentage of this type of ferry involved in this type of diasaster is huge."
Mr Osler said there was no indication that terrorism was the cause. "Bad weather is looking likely," he said, although Mr Linington pointed out that the vessel should be able to survive most sea conditions.
The Saint Catherine, another ferry travelling the same route overnight in the opposite direction, received a distress message in which the ferry captain said his ship was in danger of sinking. The agency did not say how the Saint Catherine reacted.
Mr Osler said a collision or a leak could be among the reasons for water to enter the vessel, which measures 118m (387ft) long by 23.6m (77ft) wide.
He said: "It only takes a relatively small ingress of water to set up a sort of rocking effect that gains momentum and tilts the ship. If water got on for any reason that is the sort of thing that could happen."
A structural survey last year had found there was "nothing significant to report," the company said. The ship had a stability refit in October 2003.
Mr Osler said the roll-on, roll-off type of ships were still in use in some places, but since 1987, stability had been "massively" increased. "This vessel predates all that and has been pensioned off to the Third World," he said. It had previously served as an Italian vessel from 1970 to 1999.
The company's owner, Mamdouh Ismail, said the ship was more than 25 years old and registered in Panama. It had received a safety management certificate from an Italian organisation in October 2005, covering safety drills and other on-board procedures.
The ship disappeared from radar screens shortly after sailing from the western Saudi port of Duba at 7pm local time on Thursday night. It was due to have arrived at Safaga at 3am.
Coastal stations received no Mayday message from the crew, said Adel Shukri, of el-Salam Maritime Navigation. The weather had been very poor overnight on the Saudi side of the Red Sea, he said. But visibility should have been good out at sea, he added.
* 2002: Joola, Senegal:
Almost 2,000 people died when the overcrowded Joola sank in a storm as it was sailing to Dakar.
* 1996: Bukoba, Tanzania:
The Tanzanian ferry Bukoba capsised on Lake Victoria, killing more than 800 people, many trapped in their cabins.
* 1994: Estonia, Estonia:
852 people drowned when the 15,000-tonEstonia ferry sank almost 20 miles from the Finnish island of Utoe.
* 1987: Dona Paz, Philippines: 4,375 people died when the Dona Paz passenger ferry sank after colliding with an oil tanker.
* 1987: M/S Herald of Free Enterprise, Zeebrugge, Belgium: The Herald of Free Enterprise ferry sank, killing 193 of the 539 people onboard.
* 1954: Toya Maru, Japan:
1,172 people died when the freightliner Toya Maru sank during a typhoon in the Tsugaru Strait between the islands of Hokkaido and Honshu.
* 1948: Kiangya, China:
The Kiangya steamship struck a mine and exploded on Huangpo Jiang, killing an estimated 3,920 people.
* 1912: Titanic, UK:
TheTitanic superliner struck an iceberg and sank, resulting in the deaths of 1,496 people.
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