WASHINGTON, Feb. 21 - A unanimous Supreme Court decision on Tuesday gave a small religious sect the right to keep importing a hallucinogenic tea, central to its ritual observance, that the government wants to ban as a controlled substance under federal narcotics law.

With an opinion by Chief Justice John G. Roberts Jr., the decision was one of the most significant applications of the Religious Freedom Restoration Act, a 13-year-old federal statute that requires the government to meet a demanding test before it can enforce a law in a way that creates a substantial obstacle to religious observance.

The government failed to carry its burden under the statute, Chief Justice Roberts said in an opinion that not only rejected the administration's specific arguments but also dismantled its theory of the case. The chief justice said the government's "bold argument" that the Controlled Substances Act, the basic federal narcotics law, "simply admits of no exceptions" could not be reconciled either with the religious freedom law or with administrative practice under the act itself.

For the past 35 years, he noted, the government has permitted American Indians to use peyote in their religious rituals despite the fact that peyote and its active ingredient, mescaline, are banned for general use under the Controlled Substances Act and have been found by Congress to be dangerous substances with a high potential for abuse.