Congress should not enact an across-the-board tax cut because the Reagan administration's tax and spending policy is "exceedingly expansionary" and will lead to high interest rates and a continuance of inflation, Wall Street economist Henry Kaufman said yesterday.

Recent figures, showing that the economy grew very strongly in the first three months of the year, have illustrated that "an-across-the-board tax cut is inappropriate," he said. Administration officials maintain that a slowdown will come soon if Congress does not enact the president's tax cut proposals.

Calling for a balanced budget by fiscal 1982 to avoid a "credit clash," Kaufman flatly disagreed with comments earlier yesterday by Treasury Secretary Donald T. Regan.

Regan told a group of construction officials that the way to economic recovery is through the administration's large tax cuts and that the Federal Reserve should curb money growth and thus reduce inflation.

Speaking at the National Press Club, Kaufman, an executive partner of investment bankers Salomon Brothers, predicted that inflation will go back into double digits later this year and probably will rise further next year. "We have not made the turn on inflation," he said.

He also repeated his forecast that interest rates will rise in coming months, in contrast to the administration's predictions. He said rates will continue to be high and unstable because the administration is relying too much on a tight money policy to fight inflation.

Kaufman said a balanced budget in fiscal 1982, which begins October, would "fight inflation head on." He cast doubt on administration assertions that Americans will work harder and save more as a result of the proposed across-the-board cuts in tax rates.

Kaufman said that while he supported many of the goals of the new administration, he thought fiscal policy was "exceedingly expansionary, does not pursue a course that fights inflation vigorously along the way and will place nearly all the antiinflation effort squarely on monetary policy."

Kaufman, who is highly respected on Wall Street, warned against "mechanical monetarism," which he said had led to ever higher and less predictable interest rates, while being unable on its own to fight inflation. Credit markets are now "precarious" he said, and there was a danger that "the real world will become the hostage of financial institutions" as the latter continue to create credit despite higher and higher interest rates.

Regan and others in the administration have criticized the Federal Reserve Board for allowing the money supply to grow erratically, although many experts believe that it is technically very difficult to ensure steady, smooth growth in the aggregates. The Treasury secretary yesterday said administration officials intend "to offer them [the Fed] a lot of advice" about money control.

Kaufman drew attention to the strong expansionary effects of Reagan's proposed military buildup and cautioned that this, together with tax cuts, would outweigh the effect on inflation of reducing nondefense spending.

Many economists criticized the previous administration for relying too much on money policy to fight inflation, while fiscal policy stayed expansionary. Kaufman said the present administration is set to do just that, and warned that "when the burden of restraining a headstrong economy falls heavily on monetary policy, then higher interest rates are bound to result."

As an economic recovery gets under way, it is harder to beat inflation, the banker warned. The administration is predicting that inflation will come down as economic growth speeds up in coming years, but this has rarely happened anywhere in the world.