“Marijuana has not been decriminalized in Texas, and these actions demonstrate a misunderstanding of how H.B. 1325 works,” the officials, all Republicans, wrote in a letter to prosecutors.

Kim Ogg, the Harris County district attorney and a Democrat, shot back by saying that laboratory confirmation “has long been required” to prove someone’s guilt.

Before the legislation went into effect, laboratories had to identify hairs on marijuana flowers and test for the presence of cannabinoids, a process that required just a few minutes and a test strip that turned purple when it was positive. Because the new law distinguishes between hemp and illicit marijuana, prosecutors say labs would now be required to determine the concentration of THC in the seized substance.

Dr. Stout said he has been able to identify only two labs in the country that can make the fine distinction necessary and that are accredited in Texas. Both of them are private.

Prosecutors would need to pay the labs to run the tests — sometimes hundreds of dollars for each sample — and to testify about the results at trial. Sending all of the state’s suspected marijuana to a small number of labs would likely overwhelm them, prosecutors have said, and would result in severe backlogs.

Still, many prosecutors agree with the governor and are continuing to charge and prosecute marijuana cases as usual. The district attorney in El Paso, Jaime Esparza, a Democrat, said this month that the law “will not have an effect on the prosecution of marijuana cases in El Paso” and a spokeswoman confirmed that he had not thrown out any cases because of the law.