The social media platform X, formerly known as Twitter, has threatened to sue independent researchers who documented a rise in hate speech and disinformation on the site since Elon Musk took it over last year.

A lawyer for the billionaire sent a letter on 20 July accusing the Center for Countering Digital Hate (CCDH) of making “a series of troubling and baseless claims that appear calculated to harm Twitter generally, and its digital advertising business specifically”.

Musk’s lawyer, Alex Spiro, argued that the research on the platform’s failure to monitor hate speech posted by Twitter Blue subscribers was misleading. Screenshots of the letter were uploaded to the CCDH website.

Two days before Spiro sent the letter, Musk had tweeted that the nonprofit organisation pushed censorship, calling it “truly evil”.

A lawyer for the CCDH responded on Monday that Musk’s threat was “ridiculous” and said it represented “a disturbing effort to intimidate those who have the courage to advocate against incitement, hate speech and harmful content online”.

The CCDH has heavily criticised Twitter in a series of reports, saying the company has made millions from both anti-LGBTQ+ content and from bringing back banned accounts.

Musk, a self-proclaimed free-speech advocate, has made big changes to the platform since his takeover last October.

They include reducing content moderation, replacing the existing Twitter verification system with a paid subscription service, firing a majority of the staff, and most recently rebranding the platform as X.

The CCDH researchers are not the only ones to document a sharp rise in hate speech on the platform following Musk’s acquisition of it.

A computer scientist at the University of Southern Californiaand his team also found that hate speech significantly increased on the platform after Musk acquired it. A study from the Institute for Strategic Dialogue and CASM Technology also found that antisemitism had remained “higher than ever” on Twitter.

“Elon Musk’s actions represent a brazen attempt to silence honest criticism and independent research, in the desperate hope that he can stem the tide of negative stories and rebuild his relationship with advertisers,” said Imran Ahmed, Chief Executive Officer of the Center for Countering Digital Hate, in a statement posted on the nonprofit’s website.

“Advertisers are fleeing his platform for one clear reason: Elon Musk has supported the proliferation of hate and racism on it, and he doesn’t care to stop it.

In a statement, X said that it rejected the claims made by CCDH and accused the organisation of “actively working to prevent free expression”.

This is not the first time that Musk has fired back at his critics.

Last year, he suspended the accounts of some prominent journalists who reported on his takeover and also suspended a user for using publicly available data to track his private plane.