UPDATE 1-U.S. Senate panel to vote next week on online sex-trafficking bill -sources

-sources@ (Adds details of legislation, background)

WASHINGTON, Nov 1 (Reuters) - The U.S. Senate Commerce Committee is expected to vote next week on a bipartisan proposal that would make it easier to penalize operators of websites that facilitate online sex trafficking, according to two sources familiar with the matter.

U.S. technology companies including Alphabet's Google and Facebook have long opposed legislation that would amend what is known as Section 230 of the decades-old Communications Decency Act, arguing it could thwart digital innovation and prompt endless litigation.

A spokesman for Senator John Thune, who chairs the Senate Commerce Committee, declined to comment.

The internet industry considers Section 230 a bedrock legal protection because it helps shield companies from liability for the activities of their users.

The legislation from Republican Senator Rob Portman introduced in August came after years of law-enforcement lobbying for a crackdown on the online classified site backpage.com, which is used for sex advertising.

The measure would make it easier for states and sex-trafficking victims to sue social media networks, advertisers and others that fail to keep exploitative material off their platforms.

The bill has attracted bipartisan support from about a third of the Senate. A companion measure has similar backing in the House of Representatives. Republicans control both chambers.

Technology lobbyists in recent weeks have engaged in negotiations with lawmakers supportive of the bill in an attempt to narrow its impact. (Reporting by Dustin Volz and David Shepardson; Editing by Andrew Hay and Peter Cooney)