The Thomson publishing empire, which has been trying to buy the employe-owned Bureau of National Affairs here for the past year, has increased its offer from $45 a share to "not less than $50," or $90 million.

But BNA, a publisher of specialty journals and newsletters, has again rebuffed Thomson. In a terse letter to Gordon C. Brunton, president of International Thomson Ltd., BNA Chairman John D. Stewart, said that "the interests of BNA, its shareholders and its employes are best served by preserving our independence and our present form of ownership."

Brunton, whose office is in London, had asked Stewart for a meeting to discuss the offer later this month, but Stewart said he and the other BNA directors "do not believe that any purpose would be served by the meeting you suggest. The position of our board of directors on this subject has not altered since our last communication."

"If you've seen that correspondence, you know where we stand," Stewart said. "Our position has not changed."

All BNA stock is owned by its 1,400 employes and by retired BNA workers. Its current value is about $18 per share. The corporate charter prohibits the sale of any stock to outsiders, but Thomson's strategy has been to make an offer so attractive that the stockholders would vote to change the charter.

In a letter to stockholders dated Dec. 23, BNA's directors informed them of the new Thomson offer and again urged them to reject it.

"A change in the rules of the game would produce a one-time capital gain for present shareholders," the directors said, "but it would change the rules of the game unfairly, we believe, because it would take away the opportunity of present and future employes to share in the form of organization that has served us so well in the past. We believe that BNA's future business success will be greater as an employe-owned organization than it could be in any other way."

The directors' notification to the stockholders followed receipt of a letter from Brunton relaying the new offer. In his letter, Brunton said the Thomson organization has "high respect for the Bureau of National Affairs, its publications, and its management and staff." The buyout offer, Brunton said, was "made in a friendly way," and BNA would have "substantial autonomy to run its own affairs."

It is possible that BNA employes and retirees who want to cash in on Thomson's offer will initiate a change in the corporate charter, but so far, workers at the organization said, there has been no overt movement to do so. Thomson executives have said they would not challenge a rebuff in court or pursue any takeover opposed by BNA shareholders.

BNA, which has offices at 1231 15th St. NW, has been an independent, employe-owned company since it was spun off from U.S. News and World Report in 1947.

International Thomson, a conglomerate created by Canadian publishing magnate Roy Thomson, owns hundreds of newspapers and magazines in the U.S. and Canada, North Sea oil interests, insurance companies and book publishing houses.