The Times, Britain's oldest and best-known newspaper, will be closed next March unless it can be sold before then, the Toronto-based Thomson Organization announced today.

The media and oil conglomerate has made an "irrevocable" decision to rid itself of The Times, Sunday Times and Times Literary and other supplement because of unrelenting financial losses and labor trouble, according to Thomson officials. Any of the publications that cannot be sold, they said, will be closed in March.

Among the media millionaires mentioned in the past as potential buyers of one or both of the Times newspapers and the supplements are Australian Rupert Murdoch, who owns the New York Post, among other publications in the United States, Britain and Australia; Britain's Lord Matthews of the Trafalgar House conglomerate, and Sir James Goldsmith. But only publisher Robert Maxwell of Pergamon Press said today that he was actively interested in the Times newspapers, and he added that he would need financial and union help to buy them.

Picking up a concept first explored by Times journalists during a labor dispute that shut the papers most of last year, Editor William Rees-Mogg said today he and the newspaper's reporters, editors and managers would try to form a consortium with outside investors to buy just the daily Times from Thomson. He said The Times needed to be separated from the larger circulation, weely Sunday Times to become financially viable again.

Sunday Times Editor Harold Evans said he was confident his paper would be bought by someone because it is already profitable except for recurring labor disputes that have prevented it from printing and distributing its full circulation each week. He agreed that the Sunday Times' labor problems and the daily Times' money problems could require separate solutions to keep them alive.

Evans, Rees-Mogg and many of their editors and reporters said it was "inconceivable" and "unthinkable" that either newspaper would be allowed to die. But some publishers and industry analysts said they could not see how the Times newspapers could ever be commercially viable without unprecedented management-union agreement to end disputes over manning, new technology and wildcat strikes that have plagued them and Britian's other national newspapers published in and around Fleet Street in London.

Rapidly rising costs and the inability to reduce them with new technology and productivity improvement now common in U.S. newspaper publishing have put nearly half of Fleet Street's papers in precarious financial positions and forced the merger, beginning next month, of London's two evening papers, the Standard and the News.

Calling Fleet Street "a jungle," the head of the Thomson Organization, Lord Kenneth Thomson, said today in Toronto that he had reached his decision to sell or close the Times newspapers with "great regret" and "very much hoped" they could be kept alive under new ownership. Thomson is willing to sell them separately, although he would prefer to pass them on as a group, as he had inherited them from his father.

The 195-year-old Times, the flagship of British journalism and recorder of the rise and fall of the British empire, was already failing financially when it was bought in 1966 by the late Lord Roy Thomson, who built up a vast empire of newspapers in English-speaking countries. He added the younger, formerly separate Sunday Times and sank millions of dollars into them.

The Times, with its relatively small national circulation of just over 300,000, was revived as the newspaper of Britian's establishment, its letters column still as important to public debate of both great and amusingly trivial issues as the houses of Parliament. The Sunday Times built a circulation of over 1.4 million with lively, in-depth writing and investigative reporting.

But both papers have suffered frequent production stoppages, culminating in the 11-month shutdown of the Times newspapers that ended last autumn with an expensively won agreement with the print unions for manning reductions, orderly dispute procedures and negotiations to introduce new computerized printing technology.

Thomson officials and the newspaper's editors today accused the unions of breaking this agreement repeatedly during the past year. They came close in recent days to a new agreement with the unions on technology when negotiations broke down. If that agreement had been reached and the terms of last year's contract had been observed, said Gordon Brunton, chairman of Thomson's British holdings, "this would have been a totally different picture."

Union leaders said management must share the blame for communications problems dividing them. Joe Wade, head of the dominant National Graphical Association, said the unions intended to talk more with management this week and "make represtations" to Lord Thomson in Toronto.

But Evans and Rees-Mogg said they had no doubt that Thomson will now sell or close the newspapers. Evans said Lord Thomson, who had used his growing wealth from North Sea oil investments to cover the Times newspapers' losses reaching nearly $40 million this year alone, was "disillusioned over the breaking of agreements" between management and the unions.

Rees-Mogg said tonight The Times' journalists were strongly behind his effort to put together a consortium of investors -- including Times journalists and managers "in a minority role" -- to buy The Times at a reasonable price and reach agreement with print unions in Fleet Street or elsewhere in Britain to publish the daily at a much lower cost.

He said he would go to Toronto Monday to see Lord Thomson and "convince him this is practical."

Because union journalists would be bargaining with the print unions "and because we have no extra money, no secret cards up our sleeve," Rees-Mogg said, he believes they might win the necessary support of unions faced with the alternative of losing all the jobs at The Times.

"It would be quite useless for another millionaire to come in on a white horse and just repeat the mistakes of the past" by buying The Times as is and facing the same big losses year after year, Rees-Mogg said. "I'm fed up to the teeth with subsidized journalism."