CAMBRIDGE, Mass. (AP) _ Polaroid Corp. today urged its stockholders to reject a sweetened $2.6 billion takeover bid from Roy E. Disney's Shamrock Holdings Inc.

Polaroid's board of directors described the $42-a-share offer as ''inadequate from a financial point of view and not in the best interest of Polaroid and its stockholders.''

It was the second time Polaroid rebuffed a takeover bid from Shamrock, a Burbank, Calif.-based television and radio concern.

Polaroid last month rejected an unsolicited offer from Shamrock that would have paid stockholders $40 a share plus 40 percent of the eventual award in Polaroid's $5.7 billion patent infringement lawsuit against Eastman Kodak Co.

Shamrock spokesman John Grimaldi said today that the company was withholding comment until officials reviewed Polaroid's decision.

Shamrock, which is owned by the family of Roy E. Disney, nephew of the late animator Walt Disney, had said that if its bid succeeded, it would drop Polaroid's fledgling venture into 35 mm film, sell most of the company's undeveloped real property, narrow the company's manufacturing facilities and simplify Polaroid's research and development activities on the company's core businesses.

Their offer was contingent on the invalidation of Polaroid's recent issuance of 9.7 million shares to a new employee stock ownership plan, a move Polaroid made early in its attempt to block Shamrock.

In a statement, Polaroid president and chief executive officer I. MacAllister Booth said the board was advised by its financial adviser, Shearson Lehman Hutton Inc., that the latest offer from Shamrock was inadequate.

Booth said Polaroid's own plan to restructure the company will be of greater benefit to stockholders.

''The foundation of our plan is a carefully targeted reorganization and streamlining of operations to focus more sharply on key growth areas,'' Booth said. ''This will result in a strengthening of the company's competitive posture, improved future operating prospects for the company and a 15 percent reduction in the number of Polaroid employees in the United States.''

Booth said Polaroid will save about $100 million a year as a result of its restructuring.

Shamrock has filed suit in Delaware, where Polaroid is incorporated, seeking to block the employee stock ownership plan. A trial has been set for next month.