The errors concern so-called Special Investigative Methods (BOM), such as undercover work and infiltration, which under Belgian law are strictly regulated. In the cases in question, it appears that no magistrate from the prosecutor’s office in Ghent was present when the methods were being approved – something which is required under the 2003 law.

The prisoners released last week included:

• a 28-year-old Ghent man who led a gang in a failed attempt to break an accomplice out of jail in Dendermonde in March 2007
• a heroin dealer caught in 2007 in possession of 300 kg of the drug. The investigation later led to the discovery of an equal amount of cocaine
• seven Indian men sentenced from three to five years for the trafficking of illegal aliens through Belgium to the United Kingdom
• a Geraardsbergen man sentenced to eight years for drug trafficking

The Ghent prosecutor’s office stressed that there is no doubt about the validity of the convictions and that all men are guilty. Essentially, on the question of prosecution presence at the approval of BOMs, Ghent took a different position from other jurisdictions, which the Court of Cassation later ruled was unlawful. In the two other Dutch-speaking jurisdictions, Brussels and Antwerp, magistrates ensured a prosecution magistrate was present at all proceedings.

It later became clear that the government was aware of the problem. Former justice minister Jo Vandeurzen, who resigned last month, had prepared a new law to patch up the difficulty in August last year. But the proposed change never received the approval of his ministerial colleagues and never reached the stage of being laid before parliament.

The Flemish bar, however, issued a statement last week rejecting the idea of a patch-up law, saying a first reading of the law’s provisions had given rise to “serious concerns”. At the weekend, the prosecutor-general in Ghent announced an appeal against the ruling that liberated the 10 men. Incoming justice minister Stefaan De Clerck said he hoped that might produce a change in the situation in a short time. The authorities will certainly be hoping it does: according to some experts, there are potentially hundreds of convictions that could be affected by the latest ruling.

Last week the various parties were split over where to lay the blame: on the legislators who had written what many feel to be an ambiguous statute or on the magistrates whose task it was to put it into practice. Green party Groen! blamed the law for being “written unclearly”, whereas Socialist party justice spokesman Renaat Landuyt claimed judges and lawyers had “created confusion”.

• Ghent police have apologised over another case of criminals being set free, after three men accused of taking part in a carjacking were released on procedural grounds. The suspects threatened a driver with a knife, threw him from his vehicle then proceeded to drive recklessly through Ghent, causing several road accidents.

• In an unrelated case, one of six suspects accused of forming the Belgian wing of Al Qaeda was released by a court in Brussels. The six were arrested in December and accused of preparing a suicide attack in Belgium or elsewhere. The five remaining suspects were planning to appeal against their continued detention.