A prominent fund manager is opposing the Dolan family’s bid to buy the rest of Cablevision’s common stock shares in an effort to take the cable company private.

Mario Gabelli said in a regulatory filing on Friday by one of his funds, Gamco Investors, that he plans to vote against the Dolan family’s offer during a special meeting to decide the matter on Oct. 24 during a vote by Cablevision shareholders.

Gabelli, Cablevision’s second largest shareholder with an 8 percent stake, has said he thinks the offer by the Dolan family undervalues the company by about $15 a share, or another $4 billion.

“Despite the likely challenges of FiOS, Google, a bidding war for the NY Yankees and so on and the concern of New York’s susceptibility to an economic slowdown, we believe our clients are best served by staying the course in Cablevision,” Gabelli wrote in a letter to Cablevision chairman Charles Dolan dated October 8.

The letter was posted on Gamco Investors’ Web site under a section called The Gabelli Blog.

Earlier this year, Cablevision’s board agreed to allow the company to be taken private by the Dolan family, which currently has the largest stake of common stock at 20 percent, for $36.26 a share.

The Dolan family may negotiate with Gabelli prior to next week’s vote in order to win his support for the transaction.