The U.S. Maritime Administration yesterday suffered a record $128 million default when a Houston ship operator failed to make debt payments on two ships that had been refurbished with funds backed by agency guarantees.

The default on the Jade Phoenix and Golden Phoenix -- the largest in the Maritime Administration's history -- nearly wipes out the agency's $141 million reserve fund for future bankruptcies. It also occurs at a time when a number of the ships the agency has insured are in financial trouble.

If another ship bankruptcy depletes the remaining $13 million in the fund, the Maritime Administration will have to borrow funds from the U.S. Treasury to meet its obligations.

Agency spokesman Walter Oates said the Phoenix ships went into default when Citibank, which represents the bondholders who invested in the project, demanded payment and none was forthcoming. A 30-day grace period for the scheduled $9.1 million payment expired yesterday. "We have to pay the outstanding principal and accrued interest," Oates said.

The Phoenix ships were built under the 47-year-old Title 11 program, which involves government guarantees for bonds that are sold to finance ship construction. More than $7 billion in bonds is outstanding under that program.

The Maritime Administration twice bailed out Phoenix operator C. C. Wei by making $9.2 million in debt payments when they came due last year. The agency refused to make such a payment yesterday, triggering the default. Officials at Wei's Houston-based company, Falcon Shipping Group, have declined to comment on the ships.

Yesterday's default marked the end of an ambitious plan to refurbish and rescue the ships that was approved by Maritime Administrator Harold E. Shear.

The Phoenix ships were built for the El Paso Co. as liquid natural gas tankers and completed in 1979. But the three tankers were found to have thousands of insulation cracks, rendering them useless, and Lloyd's of London agreed to a $300 million insurance settlement.

Rather than pay off the bondholders with the proceeds of the settlement, Shear agreed to use the money for Wei's plan to convert the ships to bulk cargo carriers. The work was done in a South Korean shipyard by the architectural firm of John J. McMullen. Shear had worked for another company owned by McMullen before joining the agency in 1981, but a government ethics review found nothing improper in his actions.

One of the ships was damaged in a storm and grounded. It is now one of four idled ships, which the maritime agency operates through a series of paper companies, that are facing a $219 million default later this year.

The other two Phoenix vessels were too large to be accommodated by many world ports and had trouble finding work. They spent part of their time hauling grain abroad under the Food for Peace program, which reserves half its cargo for U.S. flag ships.

Shear recently tried to sell the troubled vessels to the Navy, which would have required another expensive conversion.

Jesse Calhoon, president of the National Marine Engineers Beneficial Association, called the default "the end of the Title 11 program." He said the Maritime Administration "could have paid the bondholders off in 1981 , and it would have been a wash. Instead, they've turned it into a $120 million loss."

Agency officials blame a worldwide shipping depression for the record 438 ship bankruptcies that have cost the government $222 million in the last three years.