After complaints from business lawyers and university administrators, prosecutors on a Chinese espionage case have reversed course, acknowledging that the defendant, Chi Mak, may attempt to defend himself by arguing that the military-related reports he sent to China were in the public domain.

Mr. Mak is on trial in Santa Ana, Calif., on charges of conspiracy to export defense-related data without a license, acting as an unregistered agent for the Chinese government, and lying to investigators.

The government contends that he sent the Chinese sensitive plans for quiet submarine drive technology, electrical systems, and a new generation of Navy warships.

In a brief filed as the trial began last month, prosecutors said the law prohibited the transfer of all export-controlled technical data to China, regardless of whether the information was public at the time. “The public domain exception does not apply as a matter of law because the transfers involved China, which is subject to an arms embargo,” they wrote.

That conclusion was sharply disputed by legal advisers to exporters and academics, who said such a stance would cause havoc by requiring professors to exclude students from China and about 20 other countries when teaching subjects with potential defense applications.

The criticism was generated in part by bloggers commenting on a preview of the trial published in the New York Sun on March 23.

Just before the jury entered on Tuesday, one of Mr. Mak’s attorneys, Ronald Kaye, told Judge Cormac Carney that the government’s view had changed. “My understanding is that the government is now submitting that there is the potential for a public domain exception,” Mr. Kaye said.

A prosecutor, Craig Missakian, portrayed the government’s decision as a response to Judge Carney’s ruling that the defense could present evidence that the information was public. “That was the court’s ruling,” the prosecutor said, according to a transcript.

Mr. Missakian said that, if the defense has proof the information was public, “the government would not object to a jury instruction that asked the jury to determine whether or not the information was in the public domain.”

The prosecutor did not say what prompted the government to change its stance, but during the in-court exchange he did not dispute Mr. Kaye’s characterization of the public domain claim as “a legitimate defense.” A spokesman for the prosecution, Thom Mrozek, said his office could not comment while the trial is ongoing.

“I think the government came to its senses,” an export control lawyer who has consulted with the defense team, R. Clif Burns, said. “Once it’s out there, it’s out there. Once the Pandora’s box is opened, you can’t stuff it back in.”

An official at Stanford University who manages the school’s compliance with export rules, Steve Eisner, hailed the development. “It’s a good thing that they’re recognizing that the regulations do specifically allow information in the public domain not to be subject to export control regulations,” he said.

It is unclear how useful the concession will be to Mr. Mak. He is accused of providing three technical papers found on a disk in his brother’s carry-on luggage as the brother prepared to catch a flight to China from Los Angeles in 2005. Defense lawyers claim two of the papers were presented at public conferences of the American Society of Naval Engineers. The public domain issue may have no effect on the charges of lying and failing to register as a foreign agent.

In court yesterday, an agent with the Naval Criminal Investigative Service, Nicholas Mikus, testified that the papers on the disk seized at the airport were encrypted with a 113-letter key later found at the home of Mr. Mak’s brother, according to the Associated Press. However, on cross-examination, the Navy investigator said more sophisticated encryption programs are available for free on the Internet.