The Reagan administration has made no final decision about whether to attack a Libyan plant it claims is producing chemical weapons, but Pentagon contingency plans suggest the first-ever use of cruise missiles, sources say.

Such a strike would use a land-attack variant of the Navy's Tomahawk cruise missile, equipped with a non-nuclear warhead, that was first declared fit for use in war in March 1986.Use of the unmanned Tomahawk is "a preferred contingency" because of the missile's accuracy and the fact that no pilots would be endangered in bombing runs, the sources added.

The sources, who insisted on anonymity, agreed to discuss the Libyan situation after two Navy F-14 Tomcat jets shot down two Libyan jet fighters Wednesday in a confrontation over the Mediterranean Sea.

Defense Secretary Frank C. Carlucci, in discussing the conflict at a Pentagon briefing, flatly dismissed suggestions the F-14s were airborne as part of U.S. preparation for mounting an attack on the suspected chemical weapons plant. Libya says the plant makes pharmaceuticals, not chemical weapons.

President Reagan said last month his advisers were considering such a military strike.

The Washington Post reported in Thursday's editions that Reagan now opposes a U.S. military strike against the plant because it would cause an international furor that might harm other U.S. interests. The Post cited an unnamed senior administration official who said Reagan was heeding the advice of European allies, particularly British Prime Minister Margaret Thatcher, that a military response would be unhelpful.

The British government said Wednesday night, meanwhile, that it has confirmed beyond doubt that the plant in dispute was built to produce chemical weapons.

"We have been in contact with a number of countries to make the point that we have independent information on the Libyan chemical warfare programs," said a British Foreign Ministry spokesman who spoke on condition of anonymity. "This information shows that the plant is very large and that there is no doubt it is intended for chemical warfare production."

Meanwhile, The Los Angeles Times reported Thursday that Libya had initiated secret overtures to the United States in recent months in hopes of avoiding an armed confrontation. The Times, quoting unidentified officials, said the overtures were rejected because they failed to address U.S. complaints that Gadhafi's regime supports terrorism.

The Times sources said the most recent Libyan contact came less than 24 hours before the American F-14s shot down Libya's Soviet-built fighters Wednesday over the Mediterranean.

Carlucci said the F-14s were on a routine training flight, more than 600 miles from the Libyan plant and 70 miles off Libya's coast, when they were approached "in a hostile manner" and fired in self-defense.

"What he didn't say was that if orders came to attack that plant, we probably wouldn't use airplanes," said one defense official. "We can use the Tomahawk now."