Spanish court absolved the government of any responsibility for one of Europe's worst-ever environmental disasters, when the Prestige oil tanker went down off north-west Galicia.

The Prestige split in half and sank six days after it ran into trouble in November 2002, spilling more than 50,000 tonnes of crude oil into the Atlantic off Cape Finisterre, ruining fishing waters and beaches. nly one representative from the government stood trial, leading many to argue that the real culprits were never held responsible for their actions.

After an 11-year investigation, the Galician region's high court on Wednesday ruled the disaster was in part caused by the 26-year-old tanker's poor state of repair, but that it was impossible to establish criminal responsibility.

At the time of the accident, authorities in Spain refused to allow the vessel into port, a decision widely criticised by environmental groups. It was being dragged away into the Atlantic when it broke up, with oil washing up on beaches across northern Spain and parts of France.

The former head of Spain's merchant marine department, Jose Luis Lopez-Sors, was the sole member of the then government of the rightwing People's party (PP) in the dock. He was found not guilty of crimes against the environment.

Only the Prestige's captain, Apostolos Mangouras, was found guilty of any charge, that of "disobedience" for his response to the accident, for which he was given a nine-month suspended sentence. The tanker's chief engineer, Nikolaos Argyropoulos, was found not guilty. A co-defendant, the ship's second officer Ireneo Maloto from the Philippines, is on the run.

"Nobody knows exactly what might have been the cause of what happened, nor what would have been the appropriate response to the emergency situation created by the Prestige's breakdown," said the judge, Juan Luis Pía, in a televised ruling from a court in la Coruña, just miles from where the tanker sank.

Greenpeace described the decision on Wednesday as providing "a carte blanche to the oil industry to threaten the environment and citizens".

"The sentence shows that in Spain we are not capable of passing judgment on an environmental disaster … or defending the environment," said Maria Jose Caballero, director of Greenpeace campaigns.

In addition to failing to find anyone criminally responsible, the court did not find any civil responsibility. The damage caused by the oil spill was estimated at €4.3bn, according to the prosecution, but the only compensation ever paid out was the €22m that has been held since it was deposited in 2002 by the Prestige's London-based insurers. No further compensation will be forthcoming. "It is an extraordinary ruling," said a Greenpeace spokesperson.

After the accident, thousands of Spaniards gave up their time to go to Galicia to help clear up the oil, which affected 1,800 miles (2,900km) of shoreline and 1,177 beaches.

"The court's decision is absolutely scandalous. There are almost no words to describe it," Juan López de Uralde of the Green group Equo told the Guardian. "It is a kick in the teeth to all those people who went to help clean up the oil," said Uralde. "But we knew this trial had started badly, when the only member of the government on trial was from such a low level. We have had to wait 11 years and no one has been found responsible. It's a scandal of enormous dimensions."

Uralde argued that senior members of the current government of the rightwing People's party were responsible for the disaster, but that none of them had been brought before the court. At the time of the accident, the current prime minister, Mariano Rajoy, was spokesperson for the PP government, led by José María Aznar.

Greenpeace warned that another accident of a similar scale remained a real possibility following the court's decision, which it said revealed that "not only can the companies involved not be held responsible, but that they are protected by international law … and neither can you prove negligence or wilful misconduct by policy makers."

The court found "no fault among the defendants in the worst environmental disaster that has ever occurred in Spain, which means the real culprits were not tried", the environmental group added in a statement.

The Prestige was Greek-operated, officially registered in the Bahamas, but with a Liberian-registered corporation listed as its owner. In 2003, Spain brought a civil suit against the American Bureau of Shipping, the international classification society that had certified the Prestige was "in class" for its voyage, but the case was dismissed by a court in New York.

Mangouras, a veteran sea captain, was supported by fellow seafarers, many of whom said that not only did he save his crew but he stayed with his ship until the last minute, behaving in a model fashion.