Elon Musk has stepped up his ongoing feud with ChatGPT-maker OpenAI with a revived lawsuit against the firm, adding Microsoft as a defendant.

Mr Musk, a co-founder of the artificial intelligence (AI) company, accused it and the tech giant of operating a monopoly in an amended legal complaint on Thursday.

It follows previous lawsuits accusing the firm of breaching the principles he agreed to when he helped found it in 2015.

Microsoft declined to comment on the lawsuit.

An OpenAI spokesperson said Mr Musk’s refreshed complaint was “baseless”.

“Elon’s third attempt in less than a year to reframe his claims is even more baseless and overreaching than the previous ones,” they told the BBC.

They added that previous emails sent by Mr Musk, which it shared publicly in response to his original lawsuit, “speak for themselves”.

Thursday’s legal filing amends a lawsuit filed in a California court in August, and also includes LinkedIn co-founder Reid Hoffman being added as a defendant.

The BBC has approached Mr Hoffman for a response.

The lawsuit accuses OpenAI of having transformed from a “tax-exempt charity to a $157bn (£124bn) for-profit, market-paralysing gorgon”.

It also claims Microsoft and OpenAI used a monopoly to eliminate competitors in the AI sector – including Mr Musk’s own company, xAI.

“As a result of their unlawful actions, defendants have been unjustly enriched to the tune of hundreds of billions of dollars in value, while Mr Musk has been conned along with the public,” it says.

OpenAI was founded in 2015 with the aim of building an artificial general intelligence (AGI) – generally taken to mean AI that can perform any task a human being is capable of.

In 2019, the firm announced a new “capped profit” structure allowing it to raise money.

Microsoft made an initial $1bn investment into OpenAI shortly thereafter – increasing this to a multi-year, multi-billion dollar partnership in 2023.

The lawsuit also accuses boss Sam Altman – a named defendant in the lawsuit – of “rampant self-dealing”.

Mr Musk’s initial legal action filed in March argued the agreement had transformed it into “a closed-source de facto subsidiary” of the PC giant.

OpenAI denied his claims, countering that Mr Musk had previously backed the idea of a for-profit structure.

It said in a blog post on its website that the billionaire had at one point wanted “absolute control” of the company.

The renewed claims by Mr Musk come the same week that US President-elect Donald Trump has picked him for a role in government cost-cutting, as part of his drive to “dismantle” bureaucracy when he returns to the White House next year.