A Tunisian migrant-smuggler behind the worst maritime disaster in the Mediterranean in recent history has been found guilty of manslaughter by an Italian court and faces 18 years in jail.

About 700 people, including young children, were killed in April 2015 after Mohammed Ali Malek slammed an overcrowded fishing boat into a Portuguese merchant ship that was attempting a rescue of the passengers.

The fishing boat capsized in the dark of night. Hundreds of people who were trapped in its hull were killed and dozens of others drowned in the open sea. Only 28 people, including Malek, who was 27 at the time and was quickly identified as the boat’s skipper, survived the disaster.

Malek’s second in command, a Syrian named Bikhit Mahmud, 25, was also convicted of abetting illegal immigration and faces five years in jail. Both have been fined €9m by the court.

The 2015 incident drew international concern and condemnation, and forced European leaders to face up to the migration crisis. At an emergency summit in Brussels that was called just days later, the UK and others pledged to send more support to the Mediterranean to try to prevent the growing number of casualties. But those efforts have proven inadequate to stem the tide of death. This year about 4,700 people have been killed in their attempt to cross the Mediterranean, making it the deadliest year on record for migrants attempting to make it to Europe from Africa and the Middle East.

The doomed journey commandeered by Malek began in Libya on 18 April last year. Survivors later testified that they had been held at an illegal centre near the Libyan coast before they were rounded up and taken in small groups to the wooden fishing boat, where hundreds of people, mostly from sub-Saharan Africa, were loaded on board and forced into the hull. Survivors also told prosecutors that Malek had operated one of the small dinghies that took them to the fishing boat and later commanded it, brandishing a stick to keep the passengers in line.

Once the fishing boat hit international waters, Malek called the Italian coastguard in Rome, which in turn sought the help of the King Jacob, a Portuguese container ship sailing nearby. While the container ship’s captain told prosecutors that he sought to avoid a collision, witnesses said that Malek began to steer his boat toward the enormous ship, slamming it into the side of the large vessel. Passengers began to panic and the fishing boat capsized. It then sank within minutes.

Malek was taken into Italian custody shortly after his arrival, having been identified by survivors. The Tunisian repeatedly denied that he was culpable, saying he was just a regular passenger on the ship. In correspondence with the BBC, he claimed that the shipwreck was caused by a wave that was created by the King Jacob’s propellers.

Hours before the verdict was read in a court in Catania, Sicily, Malek repeated his claim of innocence. “I have been in Italy for two and a half years and I have a little son from an Italian [woman]. I want to marry her and [claim my son]. It’s the truth,” he told the court.

Prosecutors said in a statement released after the verdict was read that two important legal principles had been established in the aftermath of the case. First, that smugglers who were illegally trafficking people in international waters could be charged with a crime if they asked to be rescued. Second, that migrants who were rescued could be considered victims and not criminals guilty of illegal immigration.