Although the findings were immediately dismissed as "null and void" by the French Justice Minister, Jacques Toubon, and are only advisory, the report irrevocably taints Mr Juppe's image as the "Mr Clean" of French politics.
The report relates to the fact - not contested by Mr Juppe - that in 1993 he arranged for the rent on his son's already subsidised flat in central Paris to be cut by 1,000 francs (pounds 132) a month. Mr Juppe was then a deputy mayor of Paris with responsibility for the city's finances, and the flat is among several thousand owned by the city council and let at highly favourable rents to members of the Paris elite and their families.
A copy of the letter in which Mr Juppe authorised the rent reduction for his son was obtained and printed by the satirical weekly Canard Enchaine in June as part of a general expose of the housing arrangements of the Paris elite. The Paris prosecutor subsequently ruled that there was no case to answer because Mr Juppe senior had made "no personal gain".
A group of Socialist MPs referred the case in hypothetical terms, without naming Mr Juppe, to the central service for the prevention of corruption, an office set up by the Socialist government in 1993.
In his report, completed yesterday, the service's head, Bernard Challe, said that "in the case of an elected councillor who had his son's council rent reduced" there "could" be a case to answer under the parts of the criminal code relating to maladministration and corruption.
He said there did not have to be personal financial gain for an offence to have been committed; there could be "moral or family benefit".
Mr Challe's report had been keenly awaited, not only because the subject of privileged housing is high on the list of ordinary Parisians' preoccupations, but because advance information about the findings last week, leaked to Express magazine, prompted a clumsy - and highly contentious - damage limitation exercise on the part of the Justice Minister.
Mr Toubon, a former colleague of Mr Juppe's at the Paris town hall, warned Mr Challe in writing that his office had no "investigatory authority" and was subordinate to that of the public prosecutor, whose decision was final. Mr Toubon's office also released an announcement that Mr Challe had resigned, a fact denied by Mr Challe, who said he would resign, but was "in no hurry to do so".
The findings of the anti-corruption office deprive Mr Juppe of one of his last refuges: his reputation as a politician who is uncorrupted and incorruptible. His poll rating, like that of President Jacques Chirac, has dived. Last month alone it fell by 11 points to 37 per cent, the lowest recorded by any French prime minister at an equivalent point.
On the scale of French corruption, signing a letter to have the rent reduced on your son's large, cheap and superbly situated council flat is hardly in the grand league. But the very banality of the case almost makes the perceived offence worse.
"It showed us," said one Parisian, "that Juppe was just like all the rest."
In one sense, however, Mr Juppe has been unlucky. He just happened to reach the top at a time when a world of established conventions and values was coming into question. In mid-1993, when Mr Juppe signed the ill-fated letter, neither he - nor his colleagues in the Paris town hall - would probably have given the slightest thought to what he was doing.
The prerogative came with the job, and Mr Juppe, whose position as deputy mayor in charge of finances gave him enormous power, doubtless abused it less than most. He ensured that his immediate kin - son, daughter, ex-wife and half-brother - were well and cheaply lodged courtesy of the council, but such allocations would have been regarded as a duty as much as a perk. Others built empires on equivalent posts.
Now, though, things are changing. As Mr Juppe's response to the first accusations about his family's housing and his son's rent showed, he has lived in a time-warp. Asked to justify reducing his son's rent, he said - in what is now a widely quoted Paris joke - he was trying to prevent "the inflation of rent prices in the capital".
t Lyons (Reuter) - Alain Carignon, a former Gaullist cabinet minister, denied accepting gifts and services worth 21m francs (pounds 2.7m) in exchange for a water privatisation contract when he went on trial yesterday on corruption charges carrying a maximum 10-year sentence.