According to the report, which cited an unnamed Trump adviser, Trump planned to claim that Slim, a New York Times shareholder and Clinton Foundation donor, was trying to boost Hillary Clinton's campaign by damaging Trumps. The adviser said Trump could launch the attack as early as Friday, the report said.
A spokesman for Slim denied that the media mogul had interfered in the U.S. election, while New York Times publisher Arthur Sulzberger Jr. said Slim had never sought to influence the newspaper's coverage, the WSJ reported.
A Clinton campaign press secretary called Trump's claim a "deranged right-wing conspiracy theory," the report said.
The Slim family owned around 17 percent of The New York Times Co. Class A shares as of March, although the Sulzberger family controls the company, the report noted.
Since a 2005 video emerged last week of Trump making lewd comments about women and bragging about groping them without consent, several women have alleged the Republican nominee assaulted them.
Trump has denied the accusations in vehement terms and threatened to sue the various media outlets that have run stories. On Thursday, Trump's wife Melania tweeted a letter from her lawyer that asked People magazine to retract parts of story in which a writer on the magazine recounted how Trump allegedly assaulted her.
When Trump launched his campaign for president more than a year ago, he accused Mexican illegal immigrants of being rapists and drug dealers. In response, a TV company controlled by Slim canceled a planned project with Trump, Reuters reported at the time.
In an email sent Friday, Arthur Sulzberger, Jr., The New York Times publisher and chairman, told CNBC, "Carlos Slim is an excellent shareholder who fully respects boundaries regarding the independence of our journalism. He has never sought to influence what we report."
Slim and the Trump and Clinton campaigns did not respond to requests for comment.