The judicial report was commissioned by French parliamentarian Philippe de Villiers, a senior political figure in the French region of Vendee whose coastline was partly spoiled by heavy fuel oil slicks that seeped from the Prestige.
The Spanish government prevented emergency crews from going aboard the stricken tanker to try to prevent the disaster until they agreed in writing to tow it out to sea, "whereas good sense should have told them to bring it into the port of La Coruna," according to the report.
De Villiers said he would begin legal action against the Spanish government on Thursday in view of the enormity of Spain's liability revealed by the report.
The report was compiled by a French legal expert, Jean-Louis Guibert, after a judicial inquiry was opened by a French judge on January 2003.
The single-hulled, Liberian-registered Prestige got into difficulties off Spain's northwestern Galician coast on November 13 last year.
After drifting helplessly for six days it sank on November 19, leaking some 50,000 tonnes of oil into the sea in one of the world's worst ever oil spills.
Meanwhile French President Jacques Chirac said Europe must do more to protect its coasts from devastating oil spills similar to the Prestige disaster.
Chirac told ministers he was pleased with the progress made in the past months since the Prestige broke up, but he said he wanted "Europe to do more to make its voice heard," according to spokesman Jean-Francois Cope.
The EU last month adopted rules banning single-hull tankers carrying heavy fuel oil from EU ports.
The ban brings the EU in line with the United States, which restricted single-hull tankers carrying heavy-grade fuel from its waters three years after the 1989 Exxon Valdez disaster in Alaska.
But despite the European legislation, nothing can in principle prevent an old tanker passing through the English Channel between Britain and France, as long as it does not dock.