BOGOTA, JUNE 26 -- In a major setback for the war on cocaine, Colombia's Supreme Court has declared unconstitutional a law ratifying the U.S.-Colombian extradition treaty.

The action blocks the government from using the treaty to continue extraditing suspected Colombian traffickers wanted on drug charges in the United States.

"It's very bad news for us," said a U.S. Drug Enforcement Administration official in Washington. "Extradition is what drug traffickers have feared the most."

A group of billionaire Colombians controls about 75 percent of the international cocaine trade, according to law enforcement officers. One leading mafia member, Carlos Lehder, was caught in February and extradited to the United States. Other kingpins of the so-called Medellin cartel, named after the Colombian city where the cocaine business is headquartered, remain at large.

Justice Minister Jose Manuel Arias Carrizosa said the judges' decision, announced yesterday, had caused the loss of a valuable instrument in the fight against narcotics trafficking. He said government officials were studying "a series of alternatives."

While the Colombian Congress could be asked to approve a new law reactivating the extradition treaty, public support for such a measure is weak. Although most Colombians favor a crackdown on drugs, polls show that a majority resent extradition as a violation of national sovereignty.

Conceivably, too, the government might still be able to extradite Colombian nationals under a 1888 bilateral treaty, to which a drug-related amendment was added in 1940, or under a 1961 United Nations convention. But doing so would certainly open the government to legal challenges and expose it to political attack.

A switch now to some other basis for extradition poses international legal complications, experts said. The court's ruling has put Colombia at legal odds with the United States, which still considers the extradition treaty, signed in 1979, in force.

{"No matter what, international treaties stand above internal decisions and as such they are law and must be obeyed," the U.S. Embassy said in a statement on the court ruling, United Press International reported.}

Colombia recently stepped up policing efforts against traffickers following a wave of killings of judges, journalists, law enforcement officers and others. Police sweeps in the past month have uncovered vast coca plantations in the southern lowlands and central highlands, raising estimates of the amount of the plant from which cocaine is made under cultivation.

Despite a buildup in the country's 1,500-strong antinarcotics force, Colombian authorities remain far from controlling an illicit business whose profits continue to exceed the nation's main legal export, coffee.

The extradition treaty was not invoked here until 1984, when the murder of Justice Minister Rodrigo Lara, by motorcycle gunmen apparently paid by traffickers, provoked then-president Belisario Betancur to declare war on the drug trade.

Since then, 16 suspected traffickers, all but three of them Colombians, have been extradited to the United States. American authorities have handed over three people to Colombia under the same treaty provisions. The United States has filed drug-related extradition requests for another 60 or so people in Colombia.

The cocaine mafia has lobbied fiercely against the treaty, even offering on several occasions to pay off Colombia's $13 billion foreign debt if U.S. extradition requests were no longer honored.

One reason drug dealers have been determined to annul the treaty, officials said, is that U.S. authorities are viewed as less vulnerable to death threats, bribes and other pressures to which Colombian judges and law enforcement officers are constantly subjected. Supreme Court justices here now live and work under extensive security.

Until the attack by leftist guerrillas on the Supreme Court's Bogota headquarters in November 1985, the justices had rejected all legal motions against extradition, on the grounds the court had no say over foreign treaties. But the guerrilla attack, which U.S. and Colombian officials believe was financed by traffickers, killed 11 of 24 justices.

Last December, a newly constituted Supreme Court decided it could rule on the 1980 legislation ratifying the treaty and declared that law invalid. The judges said the legislation had been improperly signed into law by a government minister instead of the president.

Colombia's current president, Virgilio Barco, promptly signed the old law, on the Supreme Court's recommendation.

But 10 motions were filed challenging the constitutionality of his act. A central complaint was that the Colombian president had not asked for new ratifying legislation from Congress in order to revive the treaty after December's decision. Ruling on this point last month, the justices were split 12-12.

To resolve the tie, an outside jurist was sought. Three refused to take on the job. A fourth also tried to escape the job but was told his excuses were not acceptable. In a court vote yesterday, he sided with those declaring the law unconstitutional.