Google co-founder Sergey Brin instructed his advisers to sell his personal investments in Elon Musk’s companies in recent months after learning that he had a brief affair with his wife, according to the Wall Street Journal.

Musk flatly denied the report, calling it “third-party random hearsay” and “sub-tabloid” immediately after its publication.

Musk, the co-founder of Tesla, had an alleged liaison in early December in Miami with Nicole Shanahan, the Journal said, citing unidentified people familiar with the matter. That ended the long friendship between Musk, 51, and Brin, who helped support the electric carmaker during the 2008 financial crisis. Brin, 48, filed for divorce from Shanahan in January citing “irreconcilable differences”.

In a string of posts on Twitter, the social network he is in a dispute about buying, Musk rejected the reports and lamented what he described as a thin sex life.

“Sergey and I are friends and were at a party together last night!” he posted around Monday lunchtime, AEST. “I’ve only seen Nicole twice in three years, both times with many other people around. Nothing romantic.”

“I work crazy hours, so there just isn’t much time for shenanigans,” he said in a separate post.

The size of Brin’s personal investments in Musk’s companies isn’t known, and it’s unclear whether there have been any sales, the newspaper said.

According to the Journal, Brin gave Musk $US500,000 in funding to help boost Tesla’s production as it struggled during the GFC in 2008, while in 2015, Musk presented Brin with one of Tesla’s first all-electric sport-utility vehicles.