WASHINGTON (Reuters) - A government-run "Do Not Spam" registry would only generate more unwanted e-mail because unscrupulous marketers would treat it as a source of leads, the U.S. Federal Trade Commission said on Tuesday.

The FTC declined to endorse a no-spam registry patterned after its tremendously popular Do Not Call list that allows consumers to prevent most telemarketing calls.

While telemarketers have largely complied with the new list, e-mail marketers that already violate deceptive-business laws would be unlikely to respect a list of off-limits addresses and could simply use it to send out more spam, FTC Chairman Timothy Muris said.

"A national registry was a great solution to unwanted telemarketing calls. At this time it's not the solution to unwanted e-mail," Muris said.

Unsolicited bulk messages now account for roughly 83 percent of all e-mail traffic, according to filtering company Postini Inc.

Spammers commonly route their messages through others' computers or hijack addresses to cover their tracks, a process known as "spoofing."

Enforcement of any no-spam registry would be impossible without a way to verify that e-mail has not been spoofed, Muris said.

Large Internet providers like Yahoo Inc. (YHOO.O) and Microsoft Corp. (MSFT.O) have developed methods to authenticate e-mail and have submitted them to the standards-setting Internet Engineering Task Force for approval.

Muris said the FTC would convene an industry summit in the fall to examine authentication techniques. If they are not widely adopted by the private sector, the government might be forced to set standards of its own, he said.

Congress directed the FTC to look into a do-not-spam registry when it passed a national anti-spam law last December.

Even if the FTC wanted to set up a no-spam registry, it would take years to develop, Muris said.