A mild recession this year will reduce both business and consumer borrowing and will send interest rates down from their current record levels, the chief economist of Bankers Trust Co. predicted today.

But Donald Woolley said the economy faces so many uncertainties -- from inflation to the Middle East to the course of Federal Reserve monetary policy -- that he cannot make a "definitive" forecast of when and how fast interest rates will fall.

The prime rate -- the interest banks charge their best corporate borrowers for short-term loans -- is 15 1/4 percent at most banks today. Corporate bond rates, an indication of charges businesses must pay for money they borrow for a long time, are near 12 percent for the blue-chip companies and even higher for other corporations and utilities.

Businesses and consumers will borrow less in 1980 than they did in 1979, but the federal government will step up its borrowing, he said.

Overall, Woolley expects businesses, consumers and governments (state and local, as well as federal) to borrow about $360 billion in 1980 -- from banks, bond issues, commercial paper sales, insurance companies, savings and loan associations and the like.That will be about 6 percent less than they borrowed in 1979.

Mortgage lending will decline this year, for the first time since 1975, because high interest rates will squeeze many comsumers out of the home-buying market and because savings and loan associations will have less money to lend as depositors continue to put their funds in investments that yield more than savings accounts.

Battered by inflation and already carrying a heavy debt burden, consumers also will reduce their borrowing this year.

But Woolley said the federal government will have to step up its borrowing this year for several reasons: Farm credit agencies will increase their borrowing to finance grain purchases from farmers, a rising federal deficit will boost the Treasury's need for funds and the federal financing bank (which borrows on behalf of a number of agencies) will step up its activities.

He projected a decline in gross national product of one to 1.25 percent. He said inflation won't slow until late in the year, and in the next few months may accelerate from a 13 percent annual rate to 15 percent.