The Supreme Court on Monday refused to review an appeals court decision that made it harder to prosecute insider trading and threatens to undermine a number of convictions.

The decision is a blow to Preet Bharara, the United States attorney in Manhattan, whose office oversaw a sweeping crackdown on insider trading in the $3 trillion hedge fund industry.

In a conference call with reporters, Mr. Bharara said the ruling would make it more difficult for prosecutors to bring criminal cases when corporate executives pass on an inside tip to a friend or a relative expecting nothing special in return.

“We think there is a category of conduct that will go unpunished going forward,” he said.

As is their custom, the justices gave no reasons for turning down the case, which had been closely watched, given how seldom the high court has weighed in on the subject of insider trading.