The allegation was revealed by Twitter in an S-1 filing used by companies to register their assets with the US Securities and Exchange Commission (SEC). In the letter sent by IBM, the technology company invited Twitter's executives "to negotiate a business resolution of the allegations".
The patents in question are US Patent No 6,957,224, which covers the efficient retrieval of uniform resource locators; No 7,072,849, which protects a method for presenting advertising in an interactive service; and No 7,099,862, which relates to programmatic discovery of common contacts.
In filing, Twitter admits that its IP portfolio is smaller than that of many of its competitors, and notes that NPEs "often attempt to aggressively assert claims in order to extract value from technology companies".
But the company says of the IBM letter: "We believe we have meritorious defenses to IBM's allegations, although there can be no assurance that we will be successful in defending against these allegations or reaching a business resolution that is satisfactory to us".
Despite the accusation, Twitter said in the SEC filing that it plans to offer its stock for between $23 and $25 a share - up from the previous asking price of between $17 and $20. The increase values the company at $17 billion, compared to the previous valuation of $11 billion.