An important new cancer drug that until now could be extracted only from the bark of a rare Pacific yew tree may soon be made in factories, a leading cancer expert said.

Bruce Chabner of the U.S. National Cancer Institute said scientists at Stanford University in California had cracked the problem of synthesizing taxol, considered one of the most effective cancer drugs developed over the past decade. However, it will take time to scale up the process, he said."The major hurdle in synthesizing taxol has been overcome," Mr. Chabner told journalists Tuesday at the start of an international conference in Amsterdam on new cancer drugs.

Details of the work will be published in May, he said.

Doctors wanting to exploit the new drug face an environmental dilemma - treatment for each patient requires the bark from three full-grown trees, which grow in ancient north American forests that are refuge to the endangered Northern spotted owl.

Concern about the future of the spotted owl has already halted logging operations in parts of the United States.

Bristol-Myers Squibb Co., which has the exclusive production rights, hopes to gain approval to market taxol as a treatment for refractory ovarian cancer by mid-1992.

But Mr. Chabner said the drug, which has already been in limited clinical use for some years, was probably even more effective as a treatment for breast cancer.

Mr. Chabner said he expected that progress in extracting taxol from the trees' needles and raising trees in nurseries, as well as synthesizing the chemical, would eventually improve supplies.

But it will be a slow process.

"For the next two to three years the drug will be in short supply . . . The problem is we cannot keep pace with the expanding uses for it," he said.

Some relief may also come from a European sister drug, taxotere, already being produced from yew tree needles.

Franco Cavalli, of Bellinzona University Hospital in Switzerland, said taxotere would enter major clinical evaluation tests at seven European treatment centers in the next three or four weeks.

If all went well, taxotere could become commercially available in two years, he said.