The dead were crammed into the cabin and all were wearing lifejackets after apparently being told to stay put by ship’s officers, a South Korean navy spokesman said.
Some 183 bodies have been recovered from the Sewol after the tragedy last week. Dozens more people are missing, presumed drowned.
There were 476 people on board, with many trapped inside as the ferry listed and sank within two hours of distress signals being sent. A total of 174 passengers were rescued.
The development last night came as the South Korean government has admitted some bodies from the Sewol disaster have been wrongly identified.
It said mistakes would not happen again. There have been several reports in South Korean media of bodies going to the wrong families – with the error sometimes only discovered after the remains were taken to a funeral home.
An “action plan” released by the government’s emergency task force admitted “there have been cases where the victims were wrongly transferred”.
Remains will now be transferred to families when there is a match using DNA testing or fingerprint or dental records.
The ferry sank on 16 April on its way from the city of Incheon in the north-west of the country to the southern tourist island of Jeju. More than 80 per cent of the dead and missing are students aged 16 and 17 from a Danwon High School in the city of Ansan, south of the capital Seoul.
US president Barack Obama this week visited the Blue House, South Korea’s presidential residence, and presented president Park Geun-hye with an American flag that flew over the White House the day the ship sank.
Mr Obama said: “So many were young students with their entire lives ahead of them. I can only imagine what the parents are going through at this point, the incredible heartache.”
Eleven crew members, including the captain, have been arrested on suspicion of negligence and abandoning people in need.
The Sewol was carrying an estimated 3,608 tonnes of cargo, according to reports, which is more than three times what an inspector who examined the vessel during a redesign said it could safely carry.