The chairman of the Senate Finance Committee warned yesterday that unless the Reagan administration and Congress speed up budget negotiations, the government risks sending the wrong message to foreign markets and seriously damaging America's economic future.

Sen. Lloyd Bentsen (Tex.), who delivered the weekly Democratic radio address, said President Reagan should heed the advice of British Prime Minister Margaret Thatcher. "Mrs. Thatcher has stated that a convincing package to reduce the United States' budget deficit is a vital first step toward restoring confidence in the financial markets. Mrs. Thatcher is absolutely correct," Bentsen said.

"If the negotiations between Congress and the president fail to produce major, honest, convincing progress toward deficit reduction, America will acknowledge to the world that we lack the courage and the will to set our economic house in order," he said. "We can no longer afford to send that message of failure."

As the second week of negotiations drew to a close, a Republican plan to cut $30 billion from the federal deficit remained on the table.

The administration indicated Friday that it was softening its stance against defense spending cuts. But it also appeared, following a White House meeting with Reagan and GOP leaders, that the administration was holding firm in its refusal to rein in Social Security spending.

The Republican proposal would reduce the deficit by $30 billion in fiscal 1988, which began Oct. 1, and by $45.5 billion in fiscal 1989.

"Unless we can pick up the pace of these negotiations, we risk sending the wrong signals to world markets and jeopardizing America's economic future," Bentsen said.