A federal court jury here found former Navy intelligence analyst Samuel Loring Morison guilty today of espionage and theft for leaking three spy satellite photographs that were classified secret to a British magazine.

Morison, the 40-year-old grandson of the late famed naval historian Samuel Eliot Morison, also was convicted on separate espionage and theft charges for taking portions of two other Navy documents, both classified secret, and keeping them in an envelope at his Crofton, Md., apartment.

It was the first time that anyone has been convicted under the espionage statute for leaking documents "relating to the national defense" to the press. Congress enacted the law in 1917.

Morison could be sentenced to up to 10 years in prison and a $10,000 fine on each of the four counts. U.S. District Judge Joseph H. Young said he will pass sentence on Nov. 25, and he released Morison on the same $100,000 bond posted for him last year.

Morison stood somberly at the defense table, looking disappointed as he heard the verdicts, and later left the courthouse without commenting.

"He can't say anything," one of his attorneys, Mark H. Lynch of the American Civil Liberties Union, said. "There's a sentencing proceeding pending that's very sensitive."

Lynch said the verdict will be appealed. ACLU officials have contended that a successful prosecution in the Morison case poses a grave danger to freedom of the press, at least insofar as reporting on national security matters is concerned.

The chief prosecutor in the case, Assistant U.S. Attorney Michael Schatzow, took sharp exception to such complaints and accused the ACLU of an "orchestrated campaign" dating to last year to "try this case in the newspapers."

"I don't think it means something about reporters waking up with subpoenas in their hands in the morning," Schatzow said after the verdict. "That's not going to happen. That's ludicrous."

The prosecutor said that he hopes the case will have a deterrent effect.

"Certainly, I would hope that people who are tempted to give out in an unauthorized fashion information relating to the national defense will stop doing it," Schatzow said.

"It may be true," he said, "that if people with access to classified information follow the rules, then the press won't receive what you refer to as leaks. That's different from going after the press."

The ACLU has argued that a conviction, if sustained on appeal, could give the Reagan administration a working version of Britain's Official Secrets Act, a law that makes it a crime to disclose any government information without proper authorization.

Under the prosecution's theory, government officials would be violating the espionage and theft statutes for leaking information "relating to the national defense" and, by that same theory, members of the press would be violating those same laws in keeping the information.

Schatzow emphasized that neither Jane's Defence Weekly, to whom Morison provided the photographs taken by the KH-11 reconnaissance satellite last year, nor any of the other news organizations that reprinted the pictures were indicted.

"Nobody prosecuted Jane's," Schatzow said. "Nobody prosecuted The Washington Post . . . . Nobody prosecuted CBS. Nobody prosecuted AP the Associated Press . . . .

"In my view, this was a case about a fellow who worked for the government, who had a top secret clearance, who knew precisely what he was authorized to do and not authorized to do and who for his own venal purposes abused his position and violated his oath," Schatzow said.

The seven-day trial began after months of legal skirmishes, most of them ending in victories for the prosecution.

Citing congressional debates in 1917 and when the law was amended in 1950, Morison's lawyers claimed that the sections of the espionage statute at issue were meant to apply only in a clandestine setting, to people like "spies and saboteurs," and not to disclosures to the press.

But Judge Young said it was "conceivable that Congress, in 1950 when the statute was amended, would have considered a person who 'leaked' national security information to the press a 'saboteur' or one who would weaken the internal security of the nation, and thus subject to prosecution."

Defense lawyers tried to portray Morison as a patriotic American who wanted to alert the public to growing Soviet naval power, but the judge ruled that his motives were irrelevant.

All the government had to establish, Young held, was that Morison willfully transmitted photographs and documents "relating to the national defense" to someone "not entitled to receive" them.

By the time both sides rested, there was no dispute over the facts, only whether they amounted to criminal conduct.

The testimony showed that Morison took the three KH-11 photographs from an absent colleague's desk at Naval Intelligence Support Center headquarters in Suitland sometime in late July 1984, cut the "secret" markings from them and had them mailed to Jane's.

The magazine used the photographs for a cover story about construction of a Soviet nuclear aircraft carrier in its Aug. 11, 1984, edition.

FBI analysis of the typewriter ribbon Morison used at work showed that he had been corresponding with Jane's and, in fact, had been hoping for a full-time job with the magazine.

Morison had been the U.S. editor for Jane's Fighting Ships since the mid-1970s and had been at loggerheads for months with his NISC boss, John R. Lewis, who said he thought it "immoral" for Morison to use NISC facilities for the work, and once told him not to use even "unclassified data available within this command."

Before he sent off the photographs, it turned out, Morison also had sent Jane's a rundown on a series of devastating explosions at a Soviet naval ammunition depot at Severomorsk on the Barents Sea. Jane's Defence Weekly editor Derek Wood had a $300 check sent to him for that and other contributions in preceding months.

Morison was arrested Oct. 1, 1984, after the head of British military intelligence had retrieved the photographs from Jane's and one turned out to have Morison's thumbprint. A search of his Crofton, Md., apartment turned up portions of two so-called Weekly Wires that detailed the damage done at Severomorsk as revealed by "satellite imagery."

With all that beyond dispute, the trial boiled down to two key questions. The first was whether the information about the KH-11 satellite system that could be gleaned from the photographs and the Weekly Wires was still that "closely held" in light of past leaks.

The jurors also were asked to decide whether disclosure, again in light of past leaks, would cause "potential" damage to the United States or "potential" advantage to a foreign power.

The jury delivered its verdict shortly before 3 p.m., concluding six hours of deliberations.