The number of cases is rising and France's agriculture minister has suggested there may be a mysterious 'third way' of transmitting the disease.
Jean Glavany's comments, published in a newspaper last Saturday, have caused an angry reaction from farmers and consumers who demanded he clarify his remarks.
"I said things which French scientists have been saying for several years" Mr Glavany told LCI television afterwards.
"I said that basically the scientists are in a phase of uncertainty".
He said they had identified the two most probably ways of passing BSE, but that because cases of the disease were not tapering off as expected a third method of contamination could not be ruled out.
Another decade
Until recently, French officials were confidently predicting that BSE would be eradicated by 2001.
Surprisingly however the number of cases in the first 14 weeks of 2000 is higher than expected - 14 compared to a total of 30 for 1999.
No one has been able to explain the trend.
Now one of France's senior food safety advisers has predicted that mad cow disease will continue to be a problem for another decade.
"Every country that has BSE now will still have it in 2010", Marc Savey, the head of animal health at the French food safety agency, told the BBC.
Rogue proteins
The two known methods of spreading BSE are contaminated feed and mother-to-calf infection.
Bruno Oesch, a BSE specialist at the University of Zurich, told the French news agency AFP that several other theories are being explored.
One is that the rogue proteins which cause the disease, called prions, do not biodegrade.
Like tetanus, they could remain in the soil and infect grazing animals.
The prions could also attach themselves to tiny insects such as mites in hay, which are then eaten.
Dominique Dormont, who chairs a French committee on prion diseases, suggested that the disease could be spread by injections to cattle.
"Two major outbreaks of the disease scrapie in Britain and Italy occurred as a result of vaccinations using nerve tissue," said Mr Dormont.
The most common theory is that France has been lax in applying the tougher controls on using meat in bone meal in cattle feed, introduced in 1996.
Meanwhile, in the UK, Conservative agriculture spokesman Tim Yeo demanded Britain respond vigorously to news of the cases.
Mr Yeo said: "At last the truth about BSE is being dragged out of a reluctant French government.
"This is not the first, nor will it be the last report to admit that BSE is much more widespread in France, and in contrast to Britain, the number of cases are still rising."
Liberal Democrat spokesman Colin Breed called on France to drop its ban on British beef.
"France cannot continue to ban British beef when cases are rising in France and falling in Britain."
A spokeswoman for the Ministry of Agriculture dismissed calls for a ban, saying the UK could not "act unilaterally" in contravention of EU law.