Elon Musk has threatened to walk away from his $44bn takeover of Twitter, accusing the social media company of "thwarting" his requests to learn more about its user base.

In a letter filed with regulators, Mr Musk said he was entitled to do his own measurement of spam accounts. The letter formalises a dispute that has simmered for weeks after Mr Musk declared the deal "on hold" pending further information.

Twitter has defended its estimates. But Mr Musk has said he believes spam and fake accounts represent a far greater share than the less than 5% of daily users that Twitter reports publicly.

"As Twitter's prospective owner, Mr Musk is clearly entitled to the requested data to enable him to prepare for transitioning Twitter's business to his ownership and to facilitate his transaction financing. To do both, he must have a complete and accurate understanding of the very core of Twitter's business model - its active user base," lawyer Mike Ringler wrote in the letter.

"Based on Twitter's behaviour to date, and the company's latest correspondence in particular, Mr Musk believes the company is actively resisting and thwarting his information rights," the letter said.