Elon Musk has launched legal action to summon Twitter co-founder Jack Dorsey to appear in court as part of his legal battle with the tech company.
Mr Musk is seeking to end his $44bn deal for Twitter after alleging the firm failed to provide enough information on fake account numbers.
But the social media site is suing Mr Musk to try to force him to buy it.
The case will be heard in Delaware, US, in October unless both parties decide to settle outside of court beforehand.
Twitter hopes that a judge will order Mr Musk – who is the world’s richest man – to complete the takeover at the agreed price of $54.20 per share.
But as part of the trial preparations, lawyers for Mr Musk have called on his friend, the former boss of Twitter Mr Dorsey, in the hope that he will help support the Tesla boss’s argument that the social media company hasn’t been honest about the volume of fake accounts on its platform.
A subpoena legal writ or document summons a person to attend court or orders the submission of evidence, as records or documents, before a court.
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In July, the billionaire said he planned to walk away from his deal to buy Twitter, which prompted the company to sue him.
Mr Musk accused the company of withholding information about fake accounts, but Twitter argued that these were excuses to cover buyer’s remorse.
The site has issued its own subpoenas to people who had planned to invest in the deal with Mr Musk.
Mr Dorsey stepped down as chief executive of Twitter in November last year and tweeted in support of Mr Musk when he announced his bid to buy the company in April, saying: “Elon is the singular solution I trust. I trust his mission to extend the light of consciousness.”
Last month, a US judge ruled that Twitter’s lawsuit against Mr Musk should go to trial in October.
Earlier in August, Mr Musk sold another 7.92 million shares in Tesla, worth around $6.88bn (£5.7bn), stating that he needed the money in case he is forced to buy Twitter.
Mr Musk has countersued Twitter, claiming a third of visible Twitter accounts, assessed by his team, were fake. Using that figure the team estimated that a minimum of 10% of daily active users are bots.
But filings made by his legal team in his battle with Twitter have been questioned by leading bot researchers.
Twitter says it estimates that fewer than 5% of its daily active users are bot accounts.