By Zaheer Kachwala

(Reuters) - An activist investor pushing for a sale of "Assassin's Creed" maker Ubisoft has gathered support from 10% of the French videogame publisher's shareholders, it said in a letter on Thursday that was exclusively shared with Reuters.

AJ Investments, which has less than 1% stake in Ubisoft, said it was in talks with private equity firms for its push.

"We call on the management of Ubisoft to allow the sale of the company to third parties or private equity firms at a fair price," the Slovakia-based activist investor said in the letter, which would be made public later in the day.

AJ Investments did not name the investors in its letter and declined a Reuters request for the names of the investors.

The move comes weeks after AJ Investments called for a CEO change at Ubisoft and urged it to go private or be sold.

The company's shares have declined more than 30% so far this month after the letter from AJ Investments dated Sept. 9 and following Ubisoft's announcement that the release of the next "Assassin's Creed" game would be delayed by three months.

For the year, the stock is down about 50%, underperforming rivals such as Electronic Arts, Take-Two Interactive Software and Capcom.

The "Assassin's Creed" delay follows a poor run of form in the past two years for the family-run publisher with games such as "Skull and Bones" and "Avatar: Frontiers of Pandora", which have fallen short of the expectations of investors and analysts.

Ubisoft is run by its founders, the Guillemot family, which owns 15% of the firm, followed by Chinese gaming giant Tencent, which owns just under 10%, according to LSEG data.

The Guillemot family, Ubisoft and Tencent did not respond to requests for comment.

Ubisoft's newly released "Star Wars Outlaws" had a strong launch last month but sales have slowed since then, analysts have said.

The underperformance has sparked questions about Ubisoft's strategy of launching new licensed intellectual property at a time when gamers are sticking with already proven titles.

"The focus should be much more on existing intellectual property (IP) and leveraging the strengths they have. It's a risky time to make more than $100 million bets on new IP or licensed IP that hasn't proven itself as a game yet," said Karl Kontus, co-founder of data analytics firm Video Game Insights.

AJ Investments said it would speak with Ubisoft's management on Tuesday to discuss its proposals.

(Reporting by Zaheer Kachwala in Bengaluru; Editing by Shounak Dasgupta)