The Internal Revenue Service said Tuesday that it would allow victims of Bernard L. Madoff’s huge investment fraud to claim a tax deduction related to the bulk of their losses.

The plan represents the first time the I.R.S. has come forward with a policy on the treatment of Mr. Madoff’s victims, who include Elie Wiesel, the Holocaust survivor and Nobel laureate; Steven Spielberg, the filmmaker; and John Malkovich, the actor, among scores of other individuals, charities and universities.

The matter has been a point of debate and anxiety for the victims and their accountants, given the lack of clarity in the tax code about how it should be handled.

The plan, which applies to victims of all Ponzi schemes, is likely to provide major relief to victims of Mr. Madoff, who pleaded guilty last week to orchestrating what prosecutors say is the largest Ponzi scheme  one that exceeded $50 billion and involved 13,000 investors. It is also likely to provide clarity for victims as they prepare to file federal income tax returns by the April 15 deadline, and help the I.R.S. avoid an unwanted avalanche of amended returns from victims.