France's highest constitutional court ruled tonight that several important sections of the Socialist government's bitterly disputed nationalization program are unconstitutional.

The decision means President Francois Mitterrand and his Cabinet must make significant changes in the way they apply the centerpiece of their platform--the nationalization of eight key industries and 36 private banks. The court's decision cannot be appealed.

Although the ruling does not question the nationalization program as a whole, it refuses a plan for reimbursing stockholders and takes issue with the choice of banks to be nationalized, preventing Prime Minister Pierre Mauroy from promulgating the new laws next week as he confidently had predicted.

"This is a serious affair that requires a profound reexamination, not only judicial but also political," said Pierre Joxe, head of the Socialist Party caucus in parliament.

Mauroy's office issued a statement admitting that the ruling means delay is inevitable, "keeping the companies concerned in a waiting situation," but underlining his determination to have parliament vote the necessary changes in the law as swiftly as possible.

"The government confirms its will to proceed with the nationalizations that have been announced," Mauroy's statement said. "The Cabinet meeting next Wednesday will decide on what to do for a final vote of the law in the briefest delays."

The ruling represented a setback for Mauroy, in overall charge of the nationalizations. On Thursday, he had predicted with seeming confidence that he would get approval from the court-- called the Constitutional Council--and would be able to begin putting the nationalizations into practice next week by naming new heads for companies taken over by his government.

Conversely, the decision marks at least a symbolic victory for the right-wing and centrist opposition that had vainly fought the nationalization laws in parliament, outnumbered by a comfortable Socialist majority backed by an allied Communist Party.

The court accepted the nationalizations in principle, as expected and as pointed out by Mauroy in his statement. But by ruling some provisions unconstitutional, it legitimized some of the business community's complaints and lent strength to opposition arguments that the government is pushing ahead without enough caution.

A group of opposition parliament members had sought the court ruling in a petition filed Dec. 19, the day after the law was enacted. The group charged, among other things, that the nationalizations were unconstitutional as a whole because their public utility--a constitutional requirement--had not been proved, that stockholders were being reimbursed unfairly and that the choice of banks to be nationalized was arbitrary.

The court dismissed the first charge as political. But on the second, it agreed, saying the compensation levels were unconstitutional "because of the unsatisfactory character of the evaluation method and failure to take into account dividends on 1981 profits."

This means the government will have to revise its complicated compensation plans, counting stockholders' claims to dividends for last year's operations and payable this year. This had been a subject of hard debate within the government last fall as the plan was being worked out, with Finance Minister Jacques Delors waging a losing battle for more lenient terms.

The court also ruled unconstitutional the choice of 36 major banks for nationalization, saying an exclusion for cooperative banks was unfair if these banks were similar in size and operations to those picked for nationalization. It specifically upheld the exclusion by Mauroy's government for banks with foreign ownership or a majority of foreign ownership.

There was no immediate count on how many cooperative banks could be affected by the ruling. Joxe called for a meeting Monday of the Socialist Party caucus along with Mauroy and Secretary of State Jean le Garrec, in charge of nationalizations, to inform Socialist parliament members of the decision's effect and what can be done to overcome it.

Ironically, Joxe's father Louis is a member of the nine-member Constitutional Council that handed down the ruling. The elder Joxe was a minister in several Gaullist governments.

A third section of the law ruled unconstitutional centered on provisions allowing the government and the eight major industrial groups that are to be nationalized to negotiate sale of foreign affiliates once the nationalization in completed. This was designed to preserve good relations with foreign governments and industries, but the court said additional French government controls should be imposed on such negotiations.