The watch officer on board the Tunisian vessel Ulysse was chatting on his phone, while his counterpart on the Cypriot container ship Virginia did not attend to radar alarms, Tunisian, French and Cypriot investigators said in a report presented in Tunis.
The Tunisian vessel rammed into the Cyprus-based boat -- anchored some 30 kilometres (18 miles) off Corsica -- at full speed on the morning of October 7.
The Virginia had thrown anchor in the middle of a sea lane, the report said.
It took several days of maritime manoeuvres to disentangle the boats and pump some 520 cubic metres of propulsion fuel, which had escaped tanks.
"This accident is due to human error shared between the Tunisian boat's team and that of the Cypriot boat," said Youssef Ben Romdhane, director general of sea transport in Tunisia's commerce ministry.
"The captain of the Tunisian boat was busy... making private telephone calls. He was far from the radar screen that warns of danger. He was alone", he told AFP.
The Cypriot boat was anchored in an "inadequate" area, he added.
"According to testimony... this is the first time a ship had dropped anchor in this location... (on) a sea lane used by merchant ships," he said.
The Virginia had thrown anchor there under pressure from the ship owner, Ben Romdhane said, citing the investigation's report.
The two ships had the same insurer, which estimated damage to the two ships to be at least 13.5 million euros (15.5 million dollars), while cleaning coastal waters was provisionally estimated at 10 million euros, Ben Romdhane said.
More than 12 ships, including the French and Italian navy, were mobilised to tackle the fuel spill caused by the collision.
The mayor of French coastal town Ramatuelle lodged a complaint over pollution of Pampelonne beach, a renowned retreat in the Gulf of Saint Tropez.
A separate investigation is being carried out by France's judiciary, which is still evaluating responsibility for the accident and pollution.