Consumers have the right to sue as a group over advertising they believe misled them into buying products, a divided state Supreme Court ruled Monday in reinstating a massive suit against the tobacco industry.

The 4-3 decision rejected business arguments that, if accepted, would have virtually prohibited class-action suits for false advertising by requiring proof that every plaintiff - millions of them, in some cases - had seen an allegedly deceptive ad and relied on it to make a purchase. The court majority said that evidence is required only for the single plaintiff or small group that represents the entire class.

"This gives the consumers rights to protect themselves from fraudulent advertising," said Mark Robinson, a lawyer for the smokers who sued tobacco companies in 1997.

The ruling could make California "the class-action capital of the country," retorted William Stern, a lawyer for business organizations and a co-author of Proposition 64, a 2004 ballot measure at the heart of the case.

The suit accused the companies of waging a long advertising campaign that concealed cigarettes' addictive and harmful effects. Unlike individual suits over illnesses allegedly caused by tobacco company deception, the current suit seeks reimbursement of money spent by every Californian who bought cigarettes during the period covered by the case: June 10, 1993, to April 23, 2001.

The case was filed under California's unfair-competition law, a far-reaching statute that lets private citizens sue on behalf of the general public over illegal business practices, including deceptive advertising. The law was narrowed by the business-sponsored Prop. 64, which requires a plaintiff to show that he or she had actually been harmed by the business practice.

Prop. 64 did not say, however, how the new requirement would affect class actions, in which an individual or a small group sues on behalf of consumers in the same circumstances. The crucial question Monday was whether every member of the class must show harm from the challenged business practice, a virtual impossibility in most cases.