Abbas Settou is alive “by the grace of God”, he says. Recovering in a Tunisian hospital this weekend, it is feared he is the sole survivor of 55 men and women who set off in an inflatable boat from Libya last month hoping to reach Italy.

His fellow passengers drowned or died slowly and painfully from hunger, thirst, and exhaustion after their boat ruptured, capsized, and drifted for 15 days unseen through one of the world's busiest shipping lanes. "We threw their bodies overboard," he said. They included three members of his family.

The rescued man, who says he drank seawater to survive, was seen clinging to a jerrycan and the remains of the stricken inflatable by fishermen off the Tunisian coast. They alerted the coastguard.

According to the Eritrean's account, 55 people boarded the boat in the Libyan capital, Tripoli, and reached the Italian coast approximately 24 hours later. High winds prevented them landing and claiming asylum. The boat was forced back out to sea where the hull ruptured and the engine failed.

Unable to call for help because the satellite phone they carried was broken, they quickly became disorientated as their raft drifted further out to sea. Without fresh water, his fellow passengers, from Eritrea, Sudan and Somalia, began dying of exposure and dehydration. Their efforts to signal to boats passing in the distance seemingly failed.

Their deaths will add to the 170 the UN High Commission for Refugees estimates have been declared dead or lost at sea attempting the journey from Libya to Europe this year. That total is expected to rise as more migrants risk the journey in the crossing season between now and September, when the Med is calmest. Already, 1,300 have reached Italy, but there are concerns the turmoil of the Arab Spring will see more desperate people attempt it. The tragedy has also highlighted concerns that – contrary to international law – maritime authorities and commercial shipping are turning a blind eye to migrants in distress in the Mediterranean. Critics claim migrants in distress are being left to drift or are intercepted and "pushed back" to countries in the southern Med. Scores of ships, including a number of British flagged vessels and highly equipped naval vessels are known to have sailed through the area but none saw or came to the stricken vessels aid.

Alexander Aleinikoff, deputy high commissioner for refugees, said the deaths were a "tragedy" and called for "all vessels at sea to be on heightened alert for migrants and refugees needing rescue in the Mediterranean". The Med, he said "is one of the busiest seaways in the world, and it is imperative that the time-honoured tradition of rescue at sea be upheld".

In a notorious case last year, a boat which left Tripoli with 72 people on board washed up on a Libyan shore two weeks later with just nine survivors. Dubbed the "left-to-die" boat, an official inquiry found that no one went to the boat's aid despite an international distress call being received and its position pinpointed.

The inquiry found that Italian and Maltese authorities failed to launch a search and rescue operation, and Nato military vessels patrolling in the area to enforce the Libyan arms embargo did not respond. Survivors described how a helicopter dropped biscuits and water but did not return, and a large military vessel that came into close contact with them ignored obvious distress signals.

A Dutch parliamentarian, Senator Tineke Strik, who carried out the inquiry into the incident, said too many people were losing their lives in similar circumstances. She said: "Yet again, a dinghy with 55 people on board drifted for 15 days on the Mediterranean. This time, only one person survived. When will this ever end? It is still not safe in Libya and the boats will continue to arrive. Europe knows that. I had hoped my report on the 'left-to-die boat' would serve as an eye-opener to prevent such tragedies happening time and time again. States must never hesitate to undertake immediate action to rescue people, even if they think someone else should be responsible: every minute counts."

She added: "Governments in Europe, and not only in the countries on the southern shores of Europe, must react, and take an equal share in the protection of asylum-seekers arriving from Africa."

In her report, she questioned why help for migrant boats in distress did not arrive. "The Mediterranean is one of the busiest seas in the world, and at the same time one of the best monitored. Yet it is also the sea in which the most people disappeared. We are not talking about somewhere in the middle of the Pacific, but about the Canal of Sicily which is full of ships, with many radars and with satellite imagery available."

Ms Strik said she was still investigating what happened in the "left-to-die" incident and was seeking answers about the role of British Royal Navy ships in the area. She said she had written to the UK Defence Secretary, Philip Hammond, a second time trying to find out which British military units were in the vicinity. She has refused to rule out legal action if evidence emerges that states or individuals have ignored their international obligations to go to the aid of a boat in distress.

Boats4People says the Mediterranean is becoming a "mass grave". They warn that the situation is being made worse by the fear that anyone going to the rescue of migrants could face charges of trafficking or aiding and abetting migrants. They also claim ignored rescues are going unpunished and are "trivialised" because only non-Europeans are affected.