Financial regulators in the US are suing Elon Musk after the billionaire said he would no longer cooperate with its investigation into his purchase of Twitter, now known as X.

The Securities and Exchange Commission (SEC) asked a federal court to order him to appear for a third session of testimony about the deal.

Upon receiving a letter from Mr Musk’s lawyer, which stated he refused to appear as requested, the lawsuit was filed.

The SEC was accused of harassment by the company.

A lawyer who represents Musk wrote that “unchecked government action is dangerous and the record here is troubling. Musk declines to acquiesce to the Commission’s intrusions and will not appear as you request.”

A lawsuit filed against Musk by the SEC is the latest spat between the regulator and Musk, who once said he had “no respect” for the agency.

Musk’s $44bn purchase of X last year prompted the SEC to launch an investigation.

A filing in federal court in San Francisco says the agency is examining his 2022 stock purchases before he acquired the company outright and statements he made about them.

The SEC reported that Mr Musk gave two half-days of testimony via video conference in July after being subpoenaed. Almost half of the documents it had received regarding the case came in after those meetings, so another meeting was required.

As part of the exhibit, Mr Musk’s attorney wrote to the agency that it was “unclear why the staff needs more time to divert Mr Musk from his significant obligations to his companies and shareholders”.