NEW YORK, JAN. 28 -- International Business Machines Corp. today announced a further restructuring of the computer giant's businesses and senior management in a move that IBM Chairman John F. Akers called "as significant as any we've ever had."

The restructuring included the election of two vice chairmen to IBM's board and the creation of IBM United States, which will oversee five new businesses responsible for worldwide product development and the day-to-day operating decisions now handled by corporate headquarters.

The moves are aimed at making IBM more competitive by bringing technology to the marketplace faster and giving the company a closer working relationship with its customers.

"This is a major decentralization," Akers told a news conference. "In some senses, we now have several major IBM companies."

Asked if he had come under pressure from the IBM board to take these steps because of the company's slow turnaround, Akers said, "No, it was my idea. The work was put in place at my direction last fall."

Akers said he decided it was time to fine-tune IBM's operations last fall when the company's massive two-year restructuring program, which entailed the transfer of 20,000 employees into sales jobs and the departure of 15,000 workers under early retirement, was well on its way.

Akers characterized the board's reaction to the restructuring, which has increased IBM's sales force by 20 percent, as "positive, enthusiastic and very supportive."

In a realignment of senior management, IBM directors elected Kaspar V. Cassani and Jack D. Kuehler vice chairmen of the board and members of its executive committee.

Cassani will assume the new role of review executive for worldwide marketing and services. Kuehler will become the review executive for two new lines of business -- IBM Enterprise Systems and IBM Technology Products -- and will be responsible for maintaining product excellence and leadership in those two sectors.

Several other top management posts were reshuffled.

Akers said the management changes were "as significant as any we've ever had" and the most far-reaching since 1961, when Thomas Watson Jr. succeeded his late father as chairman. "This organization change doesn't speak at all to my potential successor," Akers said in response to a question.

IBM stock was up $1, to $113.75 a share, in composite trading on the New York Stock Exchange today.

Rumors that major changes were afoot began circulating this week following the company's acknowledgment last week that its 1987 earnings would have declined for the third consecutive year without special items.

IBM United States, which will be accountable for U.S. revenue and profit, will oversee the new businesses to make sure that the company is fully responsive to the global marketplace. It will have headquarters in Purchase, N.Y.

Terry Lautenbach, who was elected a senior vice president, has been named general manager of IBM United States and will report to Kuehler.

IBM's U.S. Marketing and Services Group, formerly called the Information Systems Group, also will report to the general manager of IBM United States.