Twitter’s board of directors has taken defensive measures to counter the billionaire Elon Musk’s offer to buy the social networking site for $ 43 billion.

The board of directors also known as the “Poison Pill” and the “Limited Term Partner Rights Plan”.

The scheme prohibits an owner from purchasing more than 15 per cent of the company’s shares, which is achieved by offering investors the opportunity to purchase additional shares at a lower price.

Twitter explained its security plan to the US Securities and Exchange Commission and issued a statement explaining that the plan was needed because of Musk’s “unsolicited, unbound acquisition of Twitter” offer.

The acquisition offer is considered “hostile” when one company wants to buy another company, contrary to the wishes of the latter management mentioned on Twitter by the board of directors of the company that owns the social networking site.

Josh White, a financial economist with the U.S. Securities and Exchange Commission, told the BBC that the “poison pill” was “one of the last lines in which security firms can defend themselves against hostile acquisitions.”

“We also call it the nuclear option,” he added, adding that it was the last option for defense.

White said Twitter’s board of directors had made it clear that “the offer was not considered appropriate for the company.”

“Because Musk said he was not ready to negotiate a higher price, Twitter’s board of directors went ahead with the poison pill,” he added.

An expert with the U.S. Securities and Exchange Commission noted that the negotiation style used by Musk was surprising because if the ultimate goal of the offer was to get Twitter, it would not be the “right way” to achieve it.

White continued, “If (Kasturi) is serious about buying, I think he should start at a price and open the door to negotiations.”

Twitter’s security plan runs until April 14 next year.

Barak Agarwal, Twitter’s chief executive, said the company would not be a “hostage” to the offer.

Elon Musk, meanwhile, said during a TED2022 meeting in Vancouver that he “did not know I could get it,” although he did not disclose any other details, insisting he had a “Plan B”.

Musk announced on Twitter earlier this month that it had acquired a 9.2 percent stake, but that did not make him the largest holder of the company’s shares.

And Vanguard Asset Management reported a 10.3 percent stake in Twitter shares in its investment fund.

Musk had previously said that Twitter restricts freedom of expression on its social networking site, and echoed those allegations in his speech at the Vancouver conference.

He noted on Twitter that the motive behind the deal was to increase freedom of expression – a US constitutional right.