American entrepreneur and world's richest man Elon Musk is willing to buy all of Twitter's shares for $54.2 each, according to a filing with the US Securities and Exchange Commission.

"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," Musk says in the filing.

"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," he said.