Mexican officials told their bankers yesterday that the sudden, steep decline in the price of oil will force the country to borrow between $8 billion and $9 billion from foreigners this year, twice the amount anticipated several weeks ago, banking sources said.

Mexican negotiators said that, for political and social reasons, there are few belt-tightening steps the debtor nation still can take to conserve the dollars it needs to pay its foreign lenders and purchase imports that are vital to keeping the Mexican economy functioning.

Starting in 1982, Mexico slashed government spending on subsidies and investment, severely curbed imports, held down money growth and took other measures to husband dollars in order to pay its debts and fight inflation. The result was a severe recession. Like most other debtor nations, the country's standard of living has fallen to 1975 levels, and unemployment and poverty are extensive.

"Until now, Mexico had always emphasized the economic side of the country in its negotiations with banks," said a commercial bank executive. "Now they are emphasizing the political limitations."

Mexico is the developing world's second-biggest debtor, with foreign loans of about $97 billion. Only Brazil has more; its foreign borrowings total about $100 billion.

Mexico's slide back into dire economic straits is considered an ominous sign by many analysts, who had hoped the 3 1/2-year-old Latin American debt crisis was easing.

Only a year ago, Mexico was considered the model debtor country -- an example of the positive consequences that occur when a country takes austerity measures in order to adapt to a world in which it no longer can borrow freely. Mexico managed a modest economic rebound in 1984, ran sizable trade surpluses and, as a reward, was given big repayment concessions by banks.

But even as Mexico was signing new loan agreements last March, the economic miracle was evaporating.

Experts blame the government for part of the new crisis. Mexican officials in 1985 encouraged an excessive election-year recovery that permitted inflation to accelerate, non-oil exports to slow and imports to rise. Then in September, its problems were exacerbated by a severe earthquake that devastated Mexico City, killing thousands and causing about $4 billion in damage.

But it is the sharp decline in oil prices that ruined Mexico's economic and financial calculations and put the once-model debtor into severe economic straits.

Industry sources said Mexican debt negotiator Angel Gurria told the country's bank lenders yesterday that Mexico expects to receive an average of $16 a barrel for its oil exports this year. World prices yesterday fell toward $15 a barrel. If that price should hold, or decline further, even Mexico's bleak prognosis may be understated.

Just a few months ago, before a price war broke out among oil-producing countries, Mexico could charge $25 or more a barrel.

Every $1 decline in the price of a barrel of oil knocks about $550 million off Mexico's export revenue. Although oil prices have been falling since Mexico triggered the Latin American debt crisis in August 1982, falling interest rates and repayment concessions by banks more than offset the decline in export receipts. Now, however, few further declines in world interest rates seem likely, while the price of oil is plummeting.

Some of the $8 billion to $9 billion shortfall between Mexico's foreign payments and its anticipated dollar receipts can be made up by borrowing from official institutions such as the World Bank and the International Monetary Fund, as well as from foreign governments.

But the bulk of it, at least $6 billion to $7 billion, probably will have to come from commercial banks, which are resistant to lending Mexico vast new amounts of money.

"The thrust of the Mexican presentation [to bank lenders] was the need for a whole new set of concession s from commercial banks," according to an executive of a major U.S. bank. But the bank executive said Gurria did not outline a specific set of proposals to the lenders, instead suggesting that banks should state what concessions they are willing to make.

If Mexico's estimates are correct, the level of concessions it will need cannot be provided unless banks are ready to lend large amounts to enable the country to remain current on its outstanding debt payments, or unless the banks are prepared to reduce the rates so much on existing loans that they will take a big loss.

Banks have basically postponed repayment of any principal until 1991, so there is no way they can ease Mexico's repayment burden without lending new money or slashing current interest rates below their own cost of deposits.

The level of concessions would be almost twice what banks could expect to earn on the approximately $70 billion that they have lent Mexico, a banker said.

"We've been pushing off the inevitable for years," said the former head of Latin American lending at a major U.S. bank. "Like any other bankrupt borrower, we're going to have to eat some of the loans we made to Mexico and other debtors."