Prime minister Jose Maria Aznar's move to increase the importance of religious education in the school curriculum is coming under attack from all sides.
The new system - to be up and running by 2004-5 - will mean that both primary and secondary pupils will have to choose between classes of Catholic catechism, taught by Vatican-selected RE teachers, or history of religion, taught by state employees.
The lessons will take up three hours a week; mathematics is given five. In secondary schools, the mark will count towards the end-of-year averages:those who fail must repeat the year. In the final year, the student's RE grade will also count towards university entrance.
In the post-Franco Spanish constitution, laid out in 1978, Spain was declared religiously neutral, and RE ceased to be obligatory in schools.
Students could choose to study other subjects instead, such as ethics or technology; the grades obtained carried no academic weight.
The new law, drawn up after much lobbying from the Vatican's Spanish Cardinal, Antonio Routo, is a step back towards the previous system which was the result of an agreement between the fascist dictator and the Vatican in 1953.
A broad range of opponents claim the change is unconstitutional and unfair.
Opposition PSOE (Socialist party) leader Jose Luis Zapatero said that it was "unacceptable that a mark in RE could decide whether or not a young person could become an architect or a doctor".
Carme Chacon, the shadow education minister, said it was unconstitutional "to make citizens declare their faith", and for religion to become an academic subject.
"The state is making faith evaluable," she added. "How can you evaluate faith?"
Most of the teachers' unions and the Association of Parents called the new measures "discriminatory and unconstitutional".
Those publicly welcoming the new measures include Educacion y Gestion, the body representing the free state-funded private schools which are largely run by religious organisations. The most reactionary of the workers'
unions, the USO, is also a supporter.
Although Zapatero is threatening to take the matter to the constitutional court, Mr Aznar will go ahead with his plans.
However, in a country which is bitterly divided over religion, the issue is certain to become one of the bigger and more polemic battlegrounds in the run-up to the general elections, scheduled for the spring of 2004.
With Zapatero's PSOE starting to overtake Aznar's conservative Partido Popular in opinion polls, the measures might be reversed before they can be put into practice.