The Toronto-based Thomson publishing empire is attempting to buy the Bureau of National Affairs, the employe-owned publisher of specialty journals that is one of Washington's major private business concerns, BNA officials confirmed yesterday.

Thomson representatives have indicated they would pay as much as $45 million, but BNA officials say they will not consider selling the business at any price.

Initial contacts were made in January, and Thomson has persisted despite repeated declarations from BNA that it is not interested.

William Beltz, BNA president, and other BNA officials said they have informed Thomson emissaries of their determination to retain local control of the company. They expressed confidence that they would prevail in any legal struggle, but did not conceal their anxiety about the unwelcome approaches from an empire whose resources dwarf BNA's.

The Thomson empire, officially known as International Thomson Organisation Ltd., is a worldwide conglomerate created by the late Roy Thomson, a rags-to-riches Canadian who was knighted when he owned the Times of London. The company owns hundreds of newspapers and magazines in the United States, Canada and Europe, oil interests in the North Sea, hotels, insurance companies and book publishers.

Thomson is reportedly offering about $45 a share for BNA stock, all of which is owned by BNA employes and retired workers and currently is valued at $18.

BNA, which has offices at 1231 25th St. NW, has been an independent, employe-owned company since it was spun off from U.S. News and World Report magazine in 1947. BNA has about 1,400 employes, of whom 65 to 70 percent own stock. The company's corporate charter prohibits the sale of any stock to outsiders.

Beltz said BNA's directors, at a meeting last Thursday, "unanimously reaffirmed the position that we are not available and we don't want to pursue it." He said attorneys retained by BNA had "assured us our articles of incorporation are defensible" under the laws of Delaware, where BNA is incorporated.

Hugh Yarrington, BNA counsel, said the restrictions on stock ownership "really cannot be challenged. Of all the kinds of ways they could be challenged, we have yet to find one under which we feel any exposure."

But he said Thomson might make an offer so attractive that it would induce the holders of a majority of shares to vote to amend the corporate charter. "We've never had this kind of situation," he said. "When we have had inquiries in the past, we indicated that BNA is not for sale on any terms, and that was always accepted. This time, it appears that Thomson is not prepared to take no for an answer."

No official of Thomson could be reached yesterday. Gordon Brunton, managing director, was said by his office in Toronto to be in England. John Gill, chief financial officer, was said to be on vacation. BNA officials said they did not know of any corporate takeover counsel or investment bankers retained by Thomson.

Beltz said the directors decided at their meeting last week to give the employes full details of all contacts between BNA and Thomson executives since January. Employes said that material had not been distributed by yesterday afternoon, and some said they were surprised to learn that the rumors they had been hearing were true.

A senior writer said most BNA workers value the current structure of the corporation enough to resist the temptation of a one-time bonanza.