Australian and British government scientists have charged that leaf and soil samples said by the United States to be possible evidence of Soviet chemical warfare in Indochina are fakes, the London Observer reported in its Sunday edition.

The findings, the Observer said, could seriously weaken U.S. arguments that the Soviets have been using toxic chemical weapons in Indochina and Afghanistan.

The British government, according to an unnamed Washington source cited by the Observer, is embarrassed at the new information, which appears to undermine the U.S. campaign to prove that the Soviets have violated treaties banning chemical and biological warfare.

The State Department will discuss the findings at a meeting here next week, the Observer said.

According to the Observer, Australian scientists made exhaustive tests of leaf and pebble samples apparently collected in northern Thailand under U.S. supervision and said to have been contaminated with toxic chemicals.

After the testing, the paper reported, the scientists said the samples apparently contained no harmful poisons and were composed mostly of yellow pollen grains held together with an unidentified binding substance.

Only a trace amount of mycotoxins--the toxic element alleged by the United States to have been present--was found, the Australian scientists said, and the pollen carrying it occurred naturally in the rain forests.

The State Department, according to the Observer, has said that while the pollen occurs naturally, it also could have been selected by the Soviets as a carrier for the toxins because it is easily inhaled and can lodge in the windpipe.