Traders once again grappled—in real time—with the difference between what is real and what is fake.

What wasn't real Tuesday: a $31 billion bid for Twitter Inc.

Twitter's stock jumped more than 8% after the online publication of a bogus article that claimed the social-media firm had received a takeover offer. The story ran on a site that was a close facsimile of the main news website run by Bloomberg LP, mimicking the media company's layout and even using a Bloomberg reporter's name in the byline. A search of Internet records shows the domain, Bloomberg.market, was registered on July 10.

Twitter's stock traded up 2.5% early in the day and then spiked in the minutes after 11:36 a.m., the time stamp on the story. The article, citing "people with knowledge of the situation," said Twitter was "working closely with bankers after receiving an offer to be bought out for $31 billion."

A Bloomberg spokesman confirmed that the article wasn't Bloomberg's. A Twitter spokesperson declined to comment.

The use of false rumors and news reports to manipulate stocks is a centuries-old ruse. The difference today is that the sheer ubiquity and amount of information that courses through markets makes it difficult for traders operating at high speeds to avoid a well-crafted hoax.

In May, shares of Avon Products Inc. soared following a news release filed with the Securities and Exchange Commission that claimed that the consumer-products company was the target of a takeover bid from "PTG Capital," an apparent allusion to private-equity firm TPG, whose boilerplate self-description was mimicked in the purported filing.

The SEC later said in a lawsuit that the false Avon report came from a 37-year-old man in Bulgaria.

The suit said that the man had manipulated stocks in a similar fashion on two previous occasions over four years.

In another unusual development, shares of Penn Virginia Corp. spiked last month after a news outlet reported that the U.S. energy producer had rejected an $8-a-share takeover offer from oil major BP PLC. People familiar with the companies have said no such offer or rejection occurred between them.

That report appears to have gained traction due to still-online documentation from the same date 13 years ago of an earlier takeover tussle involving Penn Virginia and a suitor with a name similar to BP's.

The SEC declined to comment on the Twitter situation Tuesday.

A Twitter account closely followed by traders, @OpenOutcrier, appears to have been the first to tweet the false report, linking to it at 11:39 a.m. The run-up in Twitter's stock started just before 11:39 a.m., and then it raced higher in the following minutes.

CNBC anchor Carl Quintanilla cited reports of the bid on-air shortly after @OpenOutcrier's tweet, saying "we're still trying to parse the legitimacy of the report."

Mr. Quintanilla also tweeted a link to the report but quickly followed up by pointing out misspellings and grammatical errors in the story on air and on Twitter.

Bloomberg's spokesman Ty Trippet quickly tweeted to Mr. Quintanilla that the story was fake, and CNBC followed up on air with that statement.

Mr. Trippet said that no Bloomberg outlets reported the misinformation.

Within about 10 minutes, shares had retreated to where they traded before the market reacted to the fake article.

In response to a query, an email from the @OpenOutcrier Twitter account condemned the false report but said it is the account's policy "to deliver news and rumor alerts as fast as they are made available. This may leave very little time for comprehensive vetting of any given news event; such is the balance of real-time distribution and qualified accuracy."

The person added that the account will "never originate a rumor or knowingly misrepresent information we publish. In today's instance, we continued to deliver our concerns about this information with the same speed and immediacy as the original rumor."

Mr. Quintanilla, for his part, noted the need for increasing skepticism in the digital age in a follow-up tweet Tuesday: "There's an old saying in journalism: 'If your mother says she loves you, check it out.' That was before hackers."

Paul Vigna contributed to this article.

Write to Maureen Farrell at maureen.farrell@wsj.com

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