Elon Musk has launched a bid to buy Twitter for more than $40bn and release its “extraordinary potential” to boost free speech and democracy across the world.

The Tesla chief executive and world’s richest person revealed in a regulatory filing that he had launched a hostile takeover of Twitter just days after he bought a 9.2% stake. He was subsequently offered a seat on the social media company’s board but in a last-minute twist refused to take up the position.

In a letter to Bret Taylor, Twitter’s chair, Musk said the site was not thriving as a company or a tool for improving freedom of speech, and “needs to be transformed as a private company”.

Musk said the cash offer was his “best and final offer,” according to the SEC filing, adding that if it’s not accepted he would have to reconsider his position as a shareholder.

“I invested in Twitter as I believe in its potential to be the platform for free speech around the globe, and I believe free speech is a societal imperative for a functioning democracy,” he said in a letter he sent to Twitter. “However, since making my investment I now realize the company will neither thrive nor serve this societal imperative in its current form. Twitter needs to be transformed as a private company.”

Twitter issued a statement Thursday confirming that it had received the offer. It said its board would carefully review the proposal “to determine the course of action that it believes is in the best interest of the company and all Twitter stockholders.”