Italian authorities are refusing to release the findings of an inquiry into a fatal accident involving a sister ship of the Costa Concordia.
Three crew members were killed when the Costa Europa hit a quay in Sharm el-Sheikh, Egypt, in February 2010.
Accident reports should be submitted to the International Maritime Organization (IMO), but such regulations have not been followed.
The Italian Maritime Register said the report is "strictly confidential".
The Costa Europa crashed in 2010 during a Red Sea cruise with 1,500 passengers on board.
Under regulations set by the IMO, investigations into serious shipping accidents have to be conducted by the vessel's flag state - in this case Italy.
The findings of such investigations are supposed to be sent to the IMO so that potential lessons can be learned, and if necessary, changes made to safety regulations.
But two years after the accident the Italian authorities still have not sent the report to the IMO - meaning the regulator has not been able to analyse the Costa Europa accident and assess any potential improvements to safety.
"Without an investigation report, or other information submitted by a member state, the relevant IMO body has nothing on which it can make any decisions," an IMO spokesperson told Radio 4's The Report.
Allan Graveson, senior national secretary of the maritime union Nautilus International, says the failure to deliver the report is "a manifest failure of the international regulatory system".
Mr Graveson added: "Flag states should have the capacity to carry out full and complete investigations and furnish those findings to the IMO in the interests of safety of life at sea."
At the time of the accident the Egyptian authorities said the crew were at fault, while Costa Cruises blamed bad weather.
The Costa Europa accident is currently not included on the IMO's database of serious maritime incidents, although the organisation says it does not guarantee that its database is complete as it is dependent on the reports it receives.
Among the passengers on board the Costa Europa in 2010 when the accident occurred were Chris and Ron Cleal from Guernsey.
They immediately ran onto the deck where they were met by crew soaked to the waist.
"They were speaking about it being similar to the Titanic - opening bulkhead doors and being hit by a six foot wall of water," Mr Cleal recalls.
The ship had begun to list after hitting the quay and Mr Cleal says when he asked one of the crew whether he should get his life jacket, he was told not to, and that the incident was an exercise.
The Cleals say it took an hour for the captain to inform passengers of what had happened.
During this period Mrs Cleal said she was "absolutely petrified - the crew were all coming up absolutely soaking wet".
"It was obvious that there was a big problem down below," she told the BBC.
It took five hours for the Cleals to be taken off the vessel. During that time they say the only crew they saw were entertainment personnel.
Ron Cleal was so incensed at the way the accident was handled that he wrote to the UK Maritime Accident Investigation Branch asking them to investigate.
Mr Cleal said he was "due to get feedback from the Italians, but never received anything".
The BBC contacted the Italian Maritime Register who said the investigation report into the Costa Europa incident "is strictly confidential, and at the moment is in the hands of the vessel's owner and the flag state authority".
Costa Cruises told the BBC that that it carried out an internal investigation and produced a detailed report, which it shared with the flag state authorities - in this case, the Italian coastguard in Genoa.
Costa Cruises says that as the report is an internal company document it has no intention of disclosing it.
The Report is one BBC Radio 4 Thursday, 2 February at 20:00 GMT. Listen again via the Radio 4 website or download the podcast.
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