Details are still emerging but it has spread to 42 departments across the north and south of France - about half of the country. Health authorities in Britain and France have issued no warnings to tourists although the holiday season is heading for its peak.
The cause is listeria and it is the same B4 strain that killed 31 people in Switzerland between 1983 and 1987 and 48 in California in 1985. Last year in France there were only 15 cases of this strain reported.
The French health ministry has set up an emergency team of five doctors and a vet to trace the source, without success.
The first cases were reported in early April. The French ministry has put out advertisements on television to ask doctors to look out for symptoms and warn at-risk groups to avoid certain foods.
Francoise Girard, who is co-ordinating information on the epidemic for the Ministere de la Sante, said the cause is almost certain to be a mass-produced, branded product, probably a cheese or prepared dish like pate, with national distribution, which would account for the wide, episodic spread of the outbreak.
In Britain, the Department of Health said last night: 'We are still in close contact with the French authorities to monitor the situation - the source of infection remains unknown. We would expect the French government to keep the public in France informed.'
He repeated standard department advice that pregnant women, the elderly and young children should not eat soft, ripe cheeses such as Brie and Camembert, and that cooked chicken and ready- to-eat meals should be reheated thoroughly until they are 'piping hot'.
French authorities will have difficulties detecting the source of the outbreak. Symptoms may not appear for six weeks by which time contaminated food will usually have been thrown away.
Listeria produces flu-like symptoms in most healthy adults but can be dangerous for pregnant women and the frail. It can cause spontaneous abortions.
Official French figures on the deaths have not been updated since 24 June and unconfirmed reports say they are as high as 34 dead, which would make it the second most serious confirmed outbreak of listeria poisoning.
Listeriosis was first identified in a laboratory rabbit in 1926. The bacteria are commonly found in the ground and cross into the food chain because of poor hygiene. The widespread practice of silage- making on French farms was linked in the 1970s with discoveries of bacteria in cheese and meat products.
Until 1982 it was believed the disease spread directly from animals to humans. But an outbreak in Canada provided evidence that listeriosis was almost invariably a food-borne disease. Thirty-four pregnant women and seven other adults went down with listeriosis after eating coleslaw, which was identified as the cause because samples of the food survived. Sixteen people died. The vegetables had come from a field known to have listeria traces.
Three years later in California 48 people died in the worst recorded outbreak after eating fresh Mexican cheese.
The Swiss case came to light only four years after the first deaths occurred. Seven people died in early 1987 and the cause was attributed to a pasteurised Vacherin Mont d'Or cheese. Subsequently it was revealed another 24 people had died in the region.
In Britain, the Department of Health called for Swiss cheese to be withdrawn from shops and the manufacturers withdrew all stocks.
Most health authorities now discount the traditional reaction of blaming cheese made with unpasteurised milk. Richard Lacey, professor of clinical microbiology at Leeds University, said: 'It is totally wrong to draw a distinction between the two. Even if the raw milk contains listeria, the acidity needed to make the cheese would kill it. Listeria comes from dirty working conditions. The milk does not matter.
'In most cases the listeria will have been brought about by human handling and will be found on the rind or near the surface.'
In England and Wales cases of listeriosis confirmed by laboratory investigations peaked in 1988 at 291 leading to 11 spontaneous abortions and 52 deaths.
In 1987 there were 59 deaths and 18 spontaneous abortions but only 259 cases. Since 1988, cases, deaths and spontaneous abortions have fallen, so that in 1990 (the last year for which there are full figures) there were 118 cases, seven abortions and 25 deaths. This year, there had been 47 cases by June.Reuse content