A trial that could become the most severe legal challenge to Britain's Official Secrets Act opened here today, with a senior civilian official from the Ministry of Defense pleading not guilty to charges that he leaked sensitive documents to an "unauthorized" person.

Defense lawyers admitted that the defendant, 38-year-old Clive Ponting, an assistant secretary of a department handling naval affairs, had "communicated" information to a Labor Party member of Parliament, Tam Dalyell, in July 1984, dealing with the sinking on May 2, 1982, of the Argentine cruiser Belgrano by a British submarine.

But defense counsel Bruce Laughland told the 12-member jury in London's Old Bailey criminal court that the "live issue" in this case is whether disclosures to a member of Parliament, rather than to the press, were violations of state interests under Section 2 of the 74-year-old act.

"This trial is not about spying. It is a matter of lying, misleading Parliament," Laughland said.

The Ponting case has drawn extraordinary attention from Britain's civil service, from critics who charge there is too much official secrecy here, and from government officials who say discipline must be maintained and a steady flow of leaks plugged.

The case involves the supplying by Ponting to Dalyell of two documents that contain contradictions of the official government account made to Parliament immediately after the cruiser was torpedoed, killing 386 Argentine crewmen.

Dalyell, a member of a House of Commons select committee that has been investigating the Belgrano affair for many months, has been the most persistent critic of Prime Minister Margaret Thatcher's government on the sinking. He claims the cruiser was not a threat to the British fleet in the Falkland Islands war at the time it was sunk, as was claimed, and that it was torpedoed for political rather than military reasons.

The government flatly rejects both charges.

Prosecuting attorney Roy Amlot told the jury that the two documents -- one of which was unclassified and one which bore a relatively low "confidential" classification -- did not actually damage British national security. Rather, he said, "this case involves an alleged breach of confidentiality" and a breach of trust by a civil servant.

The prosecution also claimed that the information given to Dalyell fed a misleading impression that the lawmaker had about the attack, compared with "the true position as known to Mr. Ponting."

The Official Secrets Act makes it unlawful for any civil servant to communicate any document or information, even if it has no security designation and has nothing to do with national security, to unauthorized persons. The act does not define who are unauthorized persons.

Ponting's attorney did not quarrel with an account, presented by the prosecution today, of an initial police questioning of Ponting in which he first denied and then later admitted he had sent the documents.

At first, Ponting told police: "Good God, you don't suspect me?" But then, he apologized and said: "I did this because I believed that ministers within this department were not prepared to answer legitimate questions from a member of Parliament about a matter of considerable public concern, simply in order to protect their own political position."

Much of the tension surrounding this case revolves around whether the government has become involved in what some critics have called a cover-up because it did not want to acknowledge some differences in the public account given to Parliament in May 1982 and into 1983 by Thatcher and other officials.

For example, the documents show that the Belgrano was first spotted by the submarine on May 1, rather than May 2, as Parliament was told, and that for 11 hours before it was sunk it was sailing away from the British fleet rather than closing in on it, as Parliament was told.

Thatcher and many others have said -- in recent months as the controversy grew -- that the cruiser's course was irrelevant because the ship was a threat to the British fleet and could have changed course again. She also has since acknowledged other inconsistencies in the initial description of what happened.

One of the documents Ponting sent Dalyell includes a confidential memo from another defense official to the new defense minister, Michael Heseltine, advising him not to provide the House of Commons committee with all details on changes in the Rules of Engagement covering British military action at sea and changes in the exclusion zone around the Falklands. The other is a memo for Heseltine from Ponting answering some of the questions with which Dalyell had been hounding the ministry. Heseline never used the memo.