Tesla Inc. Chief Executive Officer Elon Musk said it’s “time to break up Amazon” in a tweet Thursday, escalating a rivalry with Amazon.com Inc. CEO Jeff Bezos, another billionaire investing in space exploration.

“Time to break up Amazon. Monopolies are wrong,” Musk tweeted while tagging Bezos, the world’s wealthiest man.

Musk tweeted two days after saying he was taking a break from the social networking service.

Musk’s post came in response to a tweet from a writer who said his book titled “Unreported Truths About COVID-19 and The Lockdown” was being removed from Amazon’s Kindle publishing division for violating unspecified guidelines.

With more than 35 million followers, Musk is a prolific tweeter. He has been criticized in the past for his posts on various subjects ranging from the coronavirus outbreak to Tesla stock.

Amazon didn’t immediately respond to requests for comment.

The online retailer is among tech companies being scrutinized by federal regulators and lawmakers for the increasing size and the scope of its business.

Last year, a Space Exploration Technologies Corp. executive said Bezos’s Blue Origin was years behind in efforts to build a constellation of satellites. Musk founded SpaceX.

Musk also defended Jack Dorsey after a user criticized the Twitter CEO and called on Musk to create a new social media site.

“Jack is doing his best. I support him,” Musk said on Twitter.