Mexico has won an "agreement in principle" from foreign banks to reschedule payments of $48.5 billion in government debts over the next 14 years, Finance Minister Jesus Silva-Herzog announced here today.

"This is not a traditional agreement," Silva-Herzog said at a news conference, noting that "it breaks with orthodoxy to a considerable extent" in its stretched-out amortization scheme and in its technical mechanisms designed to reduce interest rates.

The package covers virtually "everthing that is restructurable" in the Mexican public sector debt, including $20 billion due between 1985 and 1990, $23 billion that had already been restructured and $5 billion in 1983 loans.

Instead of the huge $10 billion payments due next year, Mexico will pay a modest $250 million in 1985, and no payment over $3 billion will be due until after President Miguel de la Madrid leaves office in 1988.

In addition, interest on $43 billion of the $48.5 billion -- pegged at the U.S. prime rate -- will be exchanged for the London Interbank Rate (Libor), which historically has been slightly lower.

The terms granted Mexico set a "precedent" and will "strengthen" the bargaining position of other large Latin American debtors expected to seek similar long-term restructuring arrangements, the Mexican finance minister said.

Negotiated with a 13-bank steering committee representing the country's biggest private creditors, the rescheduling proposal still must be approved by the more than 500 foreign commercial banks owed money by Mexico. While acknowledging that Mexico anticipates some resistance to the pact's generous terms, especially from smaller banks involved primarily in short-term trade financing here, Silva-Herzog expressed confidence that the complex financial transaction will be concluded within a year.

"This is not a panacea," Silva-Herzog said. "It represents a relief, a breathing space" for the country's economic ills.

Assuming that the restructuring plan is adopted, Mexico's debt as a share of gross domestic product will drop from its present 55 percent to 32 percent in 1990, officials said.

But in absolute terms, both Mexico's debt and its servicing obligations will continue to grow. Interest payments will continue to cost the country between $10 billion and $12 billion annually, officials here said. Mexico's total foreign debt is projected to swell from its present estimated $95.9 billion to more than $115 billion by 1990, according to government figures released here today -- indicating that Mexico intends to borrow an additional $20 billion from foreign banks in the course of the present decade.

A novel aspect of the proposed agreement is a provision permitting Mexico's creditors to monitor the country's economic progress through the review of government financial documents and access to the economic reports that Mexico, like most nations, presents annually to the International Monetary Fund. Though lacking a clearly defined enforcement mechanism, this arrangement would allow bankers to suspend the restructuring accord if they believed the country to be heading anew towards "disastrous" economic circumstances, Silva-Herzog said.

Under the terms of Mexico's present three-year IMF loan accord, which expires in 1985, credit disbursements from both the fund and commercial lenders depend on the country's success in meeting previously stipulated strict fiscal and economic targets. While the commercial banks would have preferred to see Mexico enter into a "permanent agreement" with the IMF, Mexico convinced them to accept a system calling for them to be informed of, but not to supervise, the country's economic development, Silva-Herzog said.

Expected to provoke grumbling among some creditor banks is the suggested near-halving of interest rates on the $5 billion jumbo loan given to Mexico in 1983. Originally bearing interest of 2 1/8 percentage points above the U.S. prime rate or 2 1/4 points above Libor, whichever was higher, the loan now would cost Mexico just 1 1/8 points over prime, or 1 1/2 points over the Libor.

Of the remaining $43.5 billion in the restructuring accord, some $22.7 billion in loans that had been pegged to the U.S. prime rate would be tied instead to the interest rate charged for certificates of deposit, a rate "basically identical to, and sometimes slightly lower, than Libor," Mexican finance officials said. Interest on the entire $43 billion sum will be set at a spread of seven-eighths of one percentage point during the 14-year accord's first two years, rising to 1 1/8 points for the next five years and to 1 1/2 points for the final seven years.

These interest rate alterations alone will save Mexico nearly $1 billion in the next two years and some $350 million annually thereafter, Mexican officials calculate.