If a campaign launched last week by one of the country's leading national newspapers is successful, Britain could take a major step toward freer reporting of Parliamentary news.

As the system is, the Parliamentary Lobby Journalists Committee - accredited correspondents at the House of Commons - glean much of their key information from "off-the-record" briefings with the prime minister's press secretary, Bernard Ingham.These twice-daily meetings take place when Parliament is in session, with Mr. Ingham often explaining the government's views on key issues of the day.

This results in nearly all the country's newspapers carrying similar stories - as if by coincidence - with their own insights into the government's thinking, but with no better attribution than to "informed sources."

The system is being challenged by the Guardian newspaper, whose editor, Peter Preston, wants the sort of Parliamentary reporting system for Britain as is enjoyed by journalists in the United States and other democratic countries.

He has instructed his correspondents to identify Mrs. Thatcher's Downing Street office as the source of news stories and to openly quote such sources, ''whether it is a description of Mr. Pym as 'Mona Lott' or Mr. John Biffen as 'a semi-detached member of the government.' "

Mr. Preston believes the time is past when the system of non-attribution can be defended.

"The heart of it, the collective, unattributable briefings for accredited journalists operated (and orchestrated) by Downing Street, has increasingly fallen in to disrepute," the Guardian argued in an editorial last week.

In the wake of the Westland helicopter affair, when information was leaked from high sources as senior government ministers disputed whether Westland should be taken over bySikorsky or by a rival European consortium, the issue of news sources is something that deserves attention.

The Guardian believes the system in Britain leaves scope for manipulation of the press, with the government feeding reporters off-the-record information, but then remaining free to deny it if the situation alters.

"The information about government deeds, government policies and government thinking - doled out day by day by a government spokesman - is surely information in the public interest which should be available - on the record, to the public," the paper adds.

It can be especially frustrating for foreign journalists, particularly those from countries like the United States where access to top political figures is much simpler, to try and report on the government and its views in such a circumspect way.

The three main opposition parties all hold regular on-the-record briefings with the same Parliamentary lobby journalists.

The leaders of each of these parties have expressed dissatisfaction with the present system of non-attributable briefings, but Mr. Preston has written to them asking for assurances that they would continue holding attributable briefings should they come to power.

The success of the Guardian's initiative for reporting reform depends

mainly on the support it receives from other newspapers.

Julia Langdon, political editor of the Daily Mirror and the political lobby's chairwoman, has said she considers that "the time for reform has come."

The lobby committee is expected to discuss the matter this week when most of them come together to cover the Labour Party annual conference in Blackpool.

Should they agree to back the Guardian it will then be up to Mrs. Thatcher whether or not she will allow Mr. Ingham to give on-the-record briefings.

But Mr. Ingham, in a letter sent to Mr. Preston, said he has "no proposals to change existing practice."

As he pointed out, his meetings with the Parliamentary lobby journalists are held at their request, and the terms of the briefings are covered by their rules.

He did not indicate whether he would be prepared to meet if the lobby were to change those rules.

The government has until early November when the next session of Parliament begins to make its decision.