Wilmington, Delaware - Twitter's lawyers have called tech billionaire Elon Musk's decision to drop his bid to buy the site "invalid" in a letter published Monday.

The social media site appears to be laying the groundwork for a legal battle with the Tesla chief executive after he announced he was withdrawing from the $44-billion deal to buy the company on Friday.

Twitter said in a letter to Musk's legal team that his termination of the deal was, in the company's view, "invalid and wrongful," and that, contrary to Musk's claims, Twitter did not violate the acquisition agreement.

The letter argued that Musk knowingly and intentionally violated the agreement by backing out.

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As a reason for dropping his bid, Musk cited unsubstantiated claims that Twitter has significantly more fake accounts than the 5% of users it has consistently claimed.

Musk's lawyers argued Twitter had not fulfilled its obligations under the terms of the deal to provide Musk with the necessary data to verify the figures.

Twitter has already signaled its intention to take Musk to court in Delaware, where Twitter is headquartered, and where the Delaware Court of Chancery has the authority to compel Musk into completing the agreement under its original terms.