Elon Musk offers to buy Twitter for $54.20 a share.

Elon Musk has offered to buy Twitter for $54.20 a share, just weeks after he became the social media company’s largest shareholder.

Mr. Musk said this was a “best and final offer,” representing a 54 percent premium over the day before he began investing in the company in late January, according to a Securities and Exchange Commission filing. It would value the company at about $43 billion.

If the offer is not accepted, Mr. Musk said he would “need to reconsider my position as a shareholder,” according to a letter sent to Bret Taylor, Twitter’s chair, on April 13 and enclosed in the filing. “Twitter has extraordinary potential. I will unlock it.”

Twitter shares were 11 percent higher in premarket trading.

On April 4, a regulatory filing revealed that Mr. Musk, the billionaire chief executive of Tesla and SpaceX and the world’s richest man, had bought a 9.2 percent stake in Twitter. The next day, Twitter announced Mr. Musk would join its board but by the end of the week he rejected the offer.

“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,” Mr. Musk said in the letter to Mr. Taylor sent on April 13.

“However, since making my investment I now realize the company will neither thrive nor serve this societal imperative in its current form,” he wrote. “Twitter needs to be transformed as a private company.”

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