WASHINGTON, Dec. 6  Investors in real estate partnerships evade billions of dollars in federal taxes each year, even though detecting this type of cheating is easy, a former specialist in partnerships for the Internal Revenue Service said Thursday.

The states, especially New York, also lose vast sums, according to the specialist, Jerry Curnutt, who retired from the tax agency in 2000.

Mr. Curnutt, who now works from his home in Arlington, Tex., said that a statistical report released by the I.R.S. last week indicated that cheating on the reporting of real estate partnership gains was growing sharply.

“Nationally, in 2005, at least $20 billion of gain from real estate tax shelters was unreported,” he said, “and in New York State, the epicenter of the business, the figure may be as high as $5 billion.”