The Treasury Department said yesterday that technical problems will force up to a two-year delay in issuing the first redesigned U.S. currency in a half-century.

The government had planned to start issuing the newly designed bills in the fall, but Treasury officials said unforeseen problems in producing adequate supplies of the new paper would force the delay.

The changes being made in the money include adding a tiny plastic thread, and microscopic printing around the portraits. Neither change would be readily apparent without close inspection, but they were both designed to thwart a new generation of copying machines.

The Secret Service has been afraid that the advent of the new machines would spawn a new class of "casual" counterfeiters who would run off money on the spur of the moment.

The government, which spent six years and $7.5 million studying the problem, announced in 1986 that it had settled on two changes designed to make it impossible to reproduce currency, even with the most sophisticated copying machines.

One change was the addition of a clear, polyester thread woven into the paper and running vertically on the bills. The thread would be invisible until held up to the light, when printing on the thread would become visible and identify the denomination of the bill.

The other change was the addition of the words "United States of America" repeated around the portrait in print so small that it would only be visible when held under a magnifying glass.

U.S. Treasurer Katherine D. Ortega said yesterday that problems have arisen in securing adequate supplies of the paper with the woven polyester thread in the quantities needed by the government, which prints 6.5 billion bills each year.

"When they are embedding the thread into the paper, it is OK at low speed, but they can't get the quality we need at high speed," said Susan Howard, a spokeswoman for the treasurer's office.

The Bureau of Engraving and Printing had been scheduled to begin full-scale production of the new notes this spring, with the new money scheduled to start appearing in circulation in the fall. But the problems will now mean the new currency may not start appearing in circulation until the fall of 1989.