Harold E. Shear has resigned as head of the Maritime Administration following widespread criticism over agency policies that detractors say have led to a record-breaking wave of shipping bankruptcies and a severe decline in U.S. shipbuilding and the merchant marine fleet.

Shear's departure last Friday, which was not announced by the agency, came one day before the Maritime Administration was forced to pay $135.6 million to cover the biggest shipping default in its history. Shear had approved an elaborate and costly plan to rebuild the two federally insured ships that went bankrupt.

The House Merchant Marine and Fisheries Committee has summoned Shear to testify this month in a probe of his handling of the agency program that guarantees loans to U.S. shipbuilders.

Sources said Shear, 65, decided to quit after failing to receive a vote of confidence from top officials at the Transportation Department, which includes the maritime agency. A Transportation spokesman said this was "completely untrue" and that Shear was not forced to resign.

Shear told President Reagan in a letter that he has worked hard during his 3 1/2-year tenure "to reverse the steady decline of the merchant fleet and restore it as a viable national asset," and that he had "achieved positive results."

"Unfortunately, certain elements of the industry remain deeply depressed," Shear said. "Recovery from the worldwide shipping slump has been very slow. I am not sure, however, that future efforts on my part can contribute much more. I have remained on the job longer than I had planned."

Shear presided over a turbulent period in the maritime industry that has severely hurt U.S. ship operators and builders. The Reagan administration has cut back on operating subsidies to shipping firms and eliminated construction subsidies to U.S. shipyards, a move that industry officials say has shifted virtually all commercial ship construction to low-cost foreign yards.

But many officials say Shear played little part in formulating these policies.

"He was a good soldier in the sense that he did what they told him to do," said Lee Rice, president of the Shipbuilders Council of America. "This administration has had a very negative maritime policy. We've gone way downhill from where we were four years ago."

Jesse Calhoon, president of the Marine Engineers Beneficial Association, recently said of Shear: "He's ineffective, and he doesn't understand the business. It's a disaster. . . . I've never seen any agency in the disarray this one is in."

The Washington Post reported last year that Shear had approved a controversial plan to convert three defective liquid natural gas tankers to dry cargo carriers. The conversion work was done by a naval architecture firm whose president previously had employed Shear and paid him a $45,000 severance fee. A government ethics review found no improper conduct on Shear's part.

One ship was damaged in a storm, and the other two last month suffered the largest default in agency history. In the wake of $200 million in other ship defaults since 1983, this has left the agency's ship reserve fund with only $9 million to cover more than $7 billion in outstanding loan guarantees, meaning that taxpayers will have to cover future defaults.

Shear served as a director of the Navy's submarine and antisubmarine programs, vice chief of naval operations and commander in chief of Allied forces in southern Europe before leaving the service in 1980. General counsel Garrett E. Brown Jr. is the agency's acting head.