Against all expectations, French ministers decided that new safeguards agreed with London and Brussels last month did not offer sufficient protection to French consumers from possible infection with the human version of mad cow disease.
The decision was angrily rejected as "wrong" by Downing Street and is certain to cause a row that will overshadow the European Union summit begining in Helsinki tomorrow.
It came as a humiliating setback for Tony Blair, who had courted Jacques Chirac, the French President, and Lionel Jospin, the French Prime Minister at a recent Downing Street summit. The Labour Party fears that French defiance of EU law will harden British public opinion against early entry to the euro.
The beef dispute now seems certain to drag into a lengthy legal battle in the European Court - exactly the outcome that the Government wished to avoid. The French government - saying that it was "driven by a priority concern for public health" - called for an immediate resumption of negotiations on two points: wider testing of British cattle for bovine spongiform encephalopathy (BSE); and more precise labelling of British exports.
These demands seem certain to be rejected by the European Commission and the Government today. Both London and Brussels thought that they had negotiated in good faith last month a package of measures covering precisely these points, and others, which would be enough to end the two-month-old beef crisis. The European Commission will now go ahead with the second stage of its legal action against France for its stance on beef.
A committee of French BSE experts gave a non-committal response on Monday to the new safeguards agreed last month, saying that progress had been made but that "plausible" risks remained. It was thought that the experts' report was sufficiently vague to allow the French government room to take a political decision to avoid a major European crisis by lifting the ban.
But the Jospin government's political courage appears to have failed in the face of pressure from consumer groups and right-wing opposition parties. French farmers were in favour of lifting the ban.
During a two-hour meeting last night, it is believed that ministers directly involved in the dispute were split over the issue. It was only when Lionel Jospin came down on the side of public safety that the decision was taken to maintain the ban.
The French government revealed in a statement that it had decided to put the "safety of consumers first". It said that the committee of French scientists had expressed fears that some of the new measures agreed with Britain would not be put into effect immediately by the Government.
They were also concerned that scientific understanding of BSE was still "developing rapidly" and that unknown methods of transmission of the disease between cattle might still be proved to exist.
Under these circumstances, the statement said, the French government was "not in a position today to lift the embargo". It hinted, however, that it might change its mind if it could have its two areas on unease satisfied. This would need the Government to introduce an enlarged and improved system of testing cattle for mad cow disease and an immediate adoption of EU rules on the labelling of British beef and the traceability of meat exports to their farm and animal of origin.
These demands will provoke consternation in London and Brussels. The French agriculture minister, Jean Glavany, had expressed himself satisfied last month with the progress in precisely these areas.
The Labour Government will be furious that, despite being fully aware of the domestic and European consequences to Britain, the Jospin government appears to have taken the path leading to the least political resistance at home. Downing Street said last night that Mr Blair had already spoken to Mr Jospin to protest at the French decision.
A spokesman for the Prime Minister said: "We have science and the law on our side and it is regrettable that the French had ignored the science and defied the law. It now means we have to go through the courts, a process that everyone had hoped to avoid. It means too that the French are totally isolated on this issue."