WASHINGTON — The Securities and Exchange Commission, in a fundamental policy shift, said Friday that it would no longer allow defendants to say they neither admit nor deny civil fraud or insider trading charges when, at the same time, they admit to or have been convicted of criminal violations.

The change is the first time that the S.E.C. has stepped back from its longstanding practice of allowing companies to settle fraud charges by paying a fine without admitting wrongdoing. The new policy will also apply to cases where a company or an individual enters an agreement with criminal authorities to defer prosecution or to not be prosecuted as part of a settlement.

Robert Khuzami, the director of enforcement at the S.E.C., said the agency would continue to use the “neither admit nor deny” settlement process when the agency alone reached a deal with a company in a case of civil securities law violations. Those types of cases make up a large majority of S.E.C. settlements.

The commission has been sharply criticized, in federal court and on Capitol Hill, for allowing companies to repeatedly settle fraud cases without admitting or denying the charges. Until last week, that policy had been applied even when a company acknowledged the same conduct to another government agency, often the Justice Department.