Tesla boss Elon Musk on Monday disclosed a 9.2-percent stake in Twitter, worth nearly $3 billion, making him the micro-blogging site's largest shareholder and triggering a rise of more than 27 percent in the company's shares.

Musk's move, revealed in a regulatory filing, comes on the heels of his tweet that he was giving "serious thought" to building a new social media platform, while questioning Twitter's commitment to free speech.

A prolific Twitter user, Musk has over 80 million followers since joining the site in 2009.

Twitter's recent quarterly results and lower-than-expected user additions have raised questions about its growth prospects, even as it pursues big projects such as audio chat rooms and newsletters to end long-running stagnation.

"It does send a message to Twitter ... having a meaningful stake in the company will keep them on their toes, because that passive stake could very quickly become an active stake," said Thomas Hayes, managing member at Great Hill Capital LLC.

A regulatory filing on Monday showed that Musk owns 73.5 million Twitter shares, which are held by the Elon Musk Revocable Trust, of which he is the sole trustee. Vanguard is Twitter's second-biggest shareholder, with an 8.79 percent stake, according to Refinitiv data.

Twitter shares rocketed higher on the announcement, surging 27.1 percent to $49.97. The stock, which had fallen 38 percent in the past 12 months through Friday's close, on Monday added as much as $8.38 billion to its market capitalization, which now stands at $39.3 billion.

(With input from agencies)