The KB-Lux case saw charges brought against five executives of the former Kredietbank (now part of KBC), including CEO Remi Vermeiren; six executives of the Luxembourg- based sister bank KB-Lux, including CEO Damien Wigny; and three beneficiaries of the conspiracy. All 14 have now been acquitted after the evidence against them was thrown out for being tainted.

The main evidence was a list of more than 8,000 names of KB-Lux account holders, stolen in 1995 by disgruntled workers who had been laid off.There were also files explaining 300 cases in which the banks had set up special arrangements to allow clients to evade taxation. The former employees first tried to turn their files into cash by blackmailing some of the clients concerned, which landed them in jail for breach of banking confidentiality. In the end, the papers found their way into the hands of the Belgian authorities.

A court in Brussels in December 2009 ruled that the case against the 14 accused - 11 bankers and three clients - was not admissible because the evidence had been obtained illegally. It had been directly obtained from an informer who was on a blacklist signifying that investigators could not work with him.

To cover up this fact, police had arranged to "find" the list by accident in a corridor in the apartment building where the informer lived. According to the Brussels court, the investigators had been "dishonest, partisan and manipulative". Investigating magistrate Jean-Claude Leys, now advocate-general in Mons, was criticised for helping to cover the police's activities.

The appeal court was not obliged to follow the trial judge's reasoning: unlawfully obtained evidence can still be admissible under Belgian law, provided the gravity of the case justifies the irregularities. But the court appears to have considered that not to be the case.

The 14 accused were facing charges of forgery, tax fraud and conspiracy. Originally, some 41 suspects faced charges, but the number was reduced to 14 before the case came to court. Of the 8,000 account holders on the stolen list, most reached an agreement with the tax authorities to pay up and avoid prosecution.

"This is painful because it is, and remains, fraud," said Bernard Clerfayt, the secretary of state in charge of tax fraud. "Thanks to an error of procedure, tax dodgers go free."

According to Groen! spokesman Stefaan Van Hecke: "It is dismaying that the government is unable to fight tax fraud within the existing legal framework. The victim is the honest taxpayer, who has to pay for the millions that major fraudsters withhold from the taxman."

Leys, meanwhile, said that he carried out his duties "in good faith and conscience. When I hear accusations of falsifying evidence, pressurising witnesses and carrying out a dishonest investigation - all those false accusations are painful. I will carry on denying them to my last breath."

The court has not ruled on the grounds of the case, but with the rejection of the main evidence, that has little likelihood of success. An appeal might be possible to the Cassation Court, but only on procedural grounds. At the weekend, the chances of that succeeding were assessed by one expert as "practically non-existent".