The lodging of criminal charges against a senior civilian official in the Ministry of Defense for allegedly leaking sensitive documents to a member of Parliament has brought to a head the battle between the Conservative government of Prime Minister Margaret Thatcher and its critics over the extent of official secrecy in Britain today.

The immediate issue, which gets its first court airing here Oct. 9, involves prosecution of Clive Ponting, 38, an assistant secretary in the ministry. He is charged with passing to an "unauthorized person" documents that dispute the official version of the 1982 torpedoing of the Argentine cruiser Belgrano, with the loss of 368 lives.

Because the documents were provided to a member of Parliament rather than to the press, the Ponting case could be the most severe test of Britain's controversial, all-encompassing, 73-year-old Official Secrets Act. That act allows prosecution of civil servants for any unauthorized disclosure of information, even if it is not classified and has nothing to do with national security.

The case, however, also comes in the aftermath of an increasing number of leaks from within the Thatcher administration, which has raised questions about whether there are shifting attitudes within Britain's traditionally nonpartisan civil service.

Perhaps most important, it has also dramatized a paradox of British society that is increasingly the subject of heightened political rhetoric, court cases, media attention and private debate among other members of Parliament. The paradox involves what critics consider to be an unacceptable level of official secrecy in one of the oldest and otherwise most open democracies.

"This country is the most secretive democracy in the world and that secrecy is becoming dangerously obsessive," claims Shirley Williams, the president of the opposition Social Democratic Party.

A just-published book, "The Secrets File" by Des Wilson, also argues that British "secrecy is more institutionalized than in any comparable country," as is "the habit of, and instinct for, secrecy more entrenched in the psyche of the governing and managerial elite" here. Furthermore, Wilson says the British people contribute to the situation because secrecy is "so deeply entrenched in the British way of life that most people have known no other system" and thus "don't demand to know more."

Although Wilson is an activist leading a campaign for a Freedom of Information Act here along the lines of the U.S. version, others share his view. David Steel, the leader of the opposition Liberal Party, endorses it in a foreword to the book and calls for repeal of the Official Secrets Act.

An American diplomat in Washington, recently returned from service here, calls Britain, only half-jokingly, "the second most secretive government in the world." First place, he said, goes to the Soviet Union.

Another book published earlier this year called "Sources Close to the Prime Minister," authored by three journalists, has opened up another controversial aspect of the issue. It argues that, with "some honorable exceptions," much of Britain's press operates willingly under a not-for-attribution system of official briefings that provides access for reporters but preserves official secrecy and fails to produce an on-the-record accountability for the public of many key issues.

Thatcher's press secretary, Bernard Ingham, is probably one of the most powerful and influential men in British government. A former labor reporter for The Manchester Guardian, he is extremely close to Thatcher. He is blunt, candid and colorful in what he tells reporters without attribution. But, as the authors note, his name virtually is never in the papers and the vast majority of Britons probably have never heard of him.

Thatcher and her aides argue that discipline within government is essential, that leakers must be discouraged and punished and that parties who are critical of secrecy when not in power tend not to do anything about it when in power.

They have noted that the most powerful criticism of the Official Secrets Act came from an official committee of inquiry in 1972 and that subsequent Labor governments failed to act upon it.

The Ponting case, however, "may prove the watershed" for the 1911 act, according to an editorial last week in the New Law Journal.

In March, a 23-year-old clerk in the Foreign Office, Sarah Tisdall, was sentenced to six months in prison under the act for leaking to The Guardian a memorandum on the scheduled arrival date in Britain of the new U.S. cruise missiles.

That decision was highly controversial. However the prospect that a senior civil servant may also be jailed is stirring even more controversy. Moderate Conservative members of Parliament say privately that it will be the Ponting case, more than questions of Thatcher's actions in sinking the Belgrano, that will have the biggest impact in Parliament and with the public.

This is mostly because the unauthorized person to whom the documents were sent was not in the media, although they eventually showed up in the left-of-center New Statesman, but Tam Dalyell, a member of Parliament from the opposition Labor Party. He has contended for the past two years that the government had not told Parliament the full story of the sinking during the Falklands war or afterward.

Ponting has framed his own defense. "My conscience is entirely clear. In my view, a civil servant must ultimately place his loyalty to Parliament and the public interest above his obligation to the government of the day," he has said, while denying any criminal culpability.

At a conference this month of the Royal Institute for Public Administration, civil service delegates showed how distinctions are being drawn between leaks to the press and to Parliament. They voted 25-21 to support the actions of someone like Ponting, but overwhelmingly against some like Tisdall.