Speaking to Al Jazeera from Paris, Catherine Pierce also of Transparency International, said: "We had evidence against those three, we thought about adding a few more but we had no evidence against them, so we dropped the case against them, but on these three we had evidence.

"The court case has not been allowed to go ahead due to law procedures ... associations like ours should be allowed to put such a complaint in front of the court but the list of those organisations is quite restricted, so the court of appeal has said that our organisation was not admissible to put such a complaint.

"It's really more of a technical problem than the merit of what we were saying."

'Humanitarian alibis'

Denis Sassou-Nguesso, the Republic of Congo's president, Omar Bongo, the late leader of Gabon, and Teodoro Obiang, the president of Equatorial Guinea, all denied any wrongdoing.

Transparency was initially successful in May when a magistrate launched a judicial investigation into its allegations that the assets were bought with embezzled public funds.

But state prosecutors appealed to try to stop the inquiry, and the appeals court ruled in their favour.

Transparency said it would appeal to France's highest court, which will not examine the substance of the allegations but only whether Thursday's appeals court decision conforms to law.

Lawyers for Obiang welcomed the decision and accused the campaigners of using "humanitarian alibis" to mask other aims.

"This shows that attempts to use the French justice system for obscure purposes are doomed to fail," they said.

During their investigations, French police had identified eight luxury cars, worth a total of $6.2m, and one apartment in a wealthy part of Paris owned by Obiang's son Teodorin.

Suspicions raised

Police had looked into the assets of the three presidents and their close relatives in 2007 following a tip-off from Transparency.

Authorities uncovered a collection of Bugattis, Ferraris, Maseratis and other luxury cars [EPA]
The authorities uncovered dozens of bank accounts, properties in rich districts of Paris and on the Riviera, and a collection of Bugattis, Ferraris, Maybachs, Maseratis and other luxury cars.

The heads of state have denied that these were bought with embezzled public funds.

The case is delicate for France, which risks straining relations with two former colonies, Gabon and Congo Republic, and with an oil producer of growing importance, Equatorial Guinea.

On the other hand, the halting of the case will raise suspicions that some African leaders are untouchable because of French interests in their countries.

Paris mansion

Bongo died in June this year, but police documents seen by the Reuters news agency also implicate his son and successor, Ali Bongo.

The Bongo family owns 39 properties in France, mostly in exclusive districts of Paris and on the Riviera.

Bongo and Sassou-Nguesso enjoyed close ties with successive French rulers and backing from Paris at testing moments of their careers.

Sassou-Nguesso and his relatives have 24 French properties, including a Paris mansion worth $28m.

Equatorial Guinea is a former Spanish colony, but other Western powers including France have shown growing interest in it since it emerged as the third-biggest oil producer in sub-Saharan Africa, after Nigeria and Angola.