Clabir Corp. of Connecticut announced yesterday that it was abandoning a 10-month hostile takeover bid for Atlantic Research Corp., a costly battle that forced the Alexandria defense contractor into a major shakeup and restructuring earlier this year.

Clabir said its military contractor subsidiary, General Defense Corp., has agreed to sell its entire 1,139,700 shares, or 12.3 percent, of Atlantic Research's outstanding common stock, to Sequa Corp. of New York for $33.9 million, or about $30 per share.

Atlantic Research President William H. Borten expressed relief that Clabir's takeover attempt was over. But the announcement prompted speculation that Sequa may be interested in acquiring Atlantic Research, speculation that Borten dismissed.

"Sequa has confirmed to us that their purchase of Atlantic Research stock is strictly an investment," said Borten.

A Sequa spokeswoman declined to comment, except to say that the "availability of the block of Atlantic Research stock provides Sequa an opportunity to invest in a company with excellent growth prospects." Sequa is a $1.2 billion manufacturing and transportation company, with principal interests in the fields of gas turbines, jet engines, military electronics and electro-optics, graphic equipment and specialty chemicals.

Clabir's decision to sell its interest in Atlantic Research came as no surprise to analysts in view of Clabir's severe financial problems in the first half of this year caused largely by unsuccessful takeover bids.

Henry D. Clarke, Clabir's chairman and chief executive officer, said the 10-month effort to acquire Atlantic Research has "been a bit frustrating... . We were never even given the courtesy of being able to meet with the management in person."

Clarke said he was approached last week by an investment banker who asked if Clabir might sell its interest in Atlantic Research. Clarke said he was unaware of the buyer's identity until after Clabir agreed to the sale.

He said the price was "almost exactly what we paid" for the shares, and a premium over the market price, which last week was $26 to $28 a share.

Yesterday Atlantic Research closed at $29.62 1/2 per share, up $1.37 1/2, after unusually heavy trading with 574,700 shares changing hands. The purchase is subject to the waiting period required under the Hart-Scott-Rodino Antitrust Act.

Sequa officials said that, in the unlikely event that the sale runs into antitrust problems and the sale cannot be consumated by Nov. 25, Clabir can sell its shares to another party. If the shares are sold for less than $30 per share during the next year, Sequa will pay the difference to Clabir.

Borten said the sale was a "positive development" for Atlantic Research. He added that the company can now "get back to business as usual. We can now concentrate on efforts to continue the company's growth rather than spending our time fighting off undesirable hostile takover bids."

But some analysts expect Sequa to make a play for Atlantic Research. Eliot H. Benson, research director at Ferris & Co. in Washington, said, "I suspect that over time Sequa will seek the acquisition of all of Atlantic Research."

To fight the Clabir bid, Atlantic Research spent hundreds of thousands of dollars on legal fees and on developing a takeover defense. Part of that strategy included the $48.8 million purchase of Ori Group, a professional services company in Rockville. The purchase diluted Clabir's stake in the company from 14.9 percent to about 12.3 percent. The costly acquisition and disappointing profits earlier this year forced the company in April to lay off 125 employes and two of its senior executives.