Renaud Van Ruymbeke, an examining magistrate in Rennes, whose investigations have led him to cases of corruption and illicit political funding, has been given an escort by the Intervention Group of the National Police, an elite police commando unit, the ministry said. Among Mr Van Ruymbeke's cases is one against Gerard Longuet, who resigned as industry minister 10 days ago because of allegations of fraud.
In Lyons, Philippe Courroye, the magistrate investigating the affairs of Michel Noir, the city's mayor who is to face trial on fraud charges in January, and of Alain Carignon, the Gaullist former communication minister remanded in custody on 12 October, has also been given police bodyguards.
The magistrates said they had received telephoned and written threats.
The protection measures were announced as Mr Van Ruymbeke's investigations raised the possibility of implicating Pierre Mehaignerie, the centrist Justice Minister.
This concerns the way the contract was awarded for the Pont de Chevire, a big motorway bridge over the Loire outside Nantes, built by the Quillery construction firm in 1991. Rene Trager, a Nantes businessman who has provided Mr Van Ruymbeke with allegations about corrupt relations between business and the world of politics, told the magistrate that bribes amounting to 12m francs ( pounds 14.5m) were paid to secure the contract.
On Friday, Mr Van Ruymbeke questioned Michel Chauty, the Gaullist former mayor of Nantes. According to media reports, Mr Chauty said Henri Caillavet, a senator for the left-wing Radicals' Movement, which is allied with the opposition Socialist Party, had told him that the Gaullist RPR party would 'not be forgotten' if Quillery won the tender. Mr Caillavet, who said he had been an occasional legal adviser to Quillery, denied Mr Chauty's version.
Mr Chauty's allegation conforms to the pattern of much modern French corruption. Companies awarded public works contracts are alleged to have rewarded the parties of local politicians. In some cases, the politicians took a personal cut. A 1990 law on party financing was introduced to stamp out the practice.
The Nantes bridge contract was approved by the Transport, Equipment and Housing Ministry when it was headed by Mr Mehaignerie between 1986 and 1988.
Mr Mehaignerie, who said this weekend that the contract was awarded 'in the strictest legality', is now the minister overseeing progress of corruption investigations.Reuse content