UPDATE 3-U.S. government to file antitrust lawsuit against Google later on Tuesday -source

(Adds company and industry details, other potential litigation)

By Diane Bartz and David Shepardson

WASHINGTON, Oct 20 (Reuters) - The U.S. Justice Department is expected to file an antitrust lawsuit against Alphabet Inc's Google on Tuesday for allegedly breaking the law in using its market power to fend off rivals, according to a source familiar with the planning.

The lawsuit is expected to allege that Google broke the law in how it treated rivals in its internet search and advertising businesses, seeking to disadvantage them to keep its own search engine dominant and using that market power to sell more ads.

Google did not immediately respond to a request for comment.

Coming just days before the U.S. presidential election, the filing's timing could be seen as a political gesture since it fulfills a promise made by President Donald Trump to his supporters to hold certain companies to account for allegedly stifling conservative voices.

A federal lawsuit would mark a rare moment of agreement between the Trump administration and progressive Democrats. U.S. Senator Elizabeth Warren tweeted on Sept. 10, using the hash tag #BreakUpBigTech, that she wanted "swift, aggressive action."

And more lawsuits could be in the offing since probes by state attorneys general into Google's broader businesses are under way, as well as an investigation of its broader digital advertising businesses. A group of attorneys general led by Texas is expected to file a separate lawsuit focused on digital advertising as soon as November, while a group led by Colorado is contemplating a more expansive lawsuit against Google.

The lawsuit comes more than a year after the Justice Department and Federal Trade Commission began antitrust investigations into four big tech companies: Amazon.com Inc , Apple Inc, Facebook Inc and Google. (Reporting by David Shepardson and Diane Bartz in Washington Editing by Edward Tobin and Matthew Lewis)