(Reuters) -A labor union-backing shareholder wrote a letter to the U.S. Securities and Exchange Commission on Friday, saying that Starbucks has failed to disclose costs arising from anti-union campaigns, which was estimated at about $240 million.

The Strategic Organizing Center (SOC), a coalition of North American labor unions, said the company "needs to immediately provide full disclosure of the total costs and liabilities ... for... informed voting decisions before the 2024 annual meeting."

Unionization efforts at the world's largest coffeehouse chain have intensified since late 2021.

More recently, workers walked out during a promotional event in November in a strike organized by the Workers United union demanding, improved staffing and schedules.

"Counter to claims made by the Strategic Organizing Center, Starbucks does not spend money on 'anti-union activities' and continues to comply with all current SEC and OLMS (Office of Labor-Management Standards) reporting requirements," a Starbucks spokesperson told Reuters in an emailed statement.

Starbucks' annual meeting, where shareholders will vote on the composition of its eight-member board, is scheduled for March 13.

The SOC, which nominated three candidates to the board of Starbucks in November, said the costs include litigation, the employee lost time, and liabilities associated with alleged labor law violations.

The U.S. Supreme Court last month agreed to hear Starbucks' appeal against a lower court ruling that it needed to rehire seven employees at one of its cafes in Memphis, Tennessee who a federal agency determined were fired for supporting unionization.

The SEC did not immediately respond to Reuters' requests for comment.

(Reporting by Savyata Mishra and Deborah Mary Sophia in Bengaluru; Additional reporting by Juveria Tabassum; Editing by Maju Samuel and Tasim Zahid)