An independent proxy adviser has recommended that shareholders of Six Flags Inc. wait for the results of a proposed sale of the company before deciding whether to support Redskins owner Daniel M. Snyder's bid for control.

However, the report by Vienna-based Proxy Governance Inc. also endorsed Snyder's ideas for reviving the company and said shareholders should turn Six Flags over to him if a buyer with a better offer does not emerge by early December.

The firm promised a follow-up report by Dec. 16, after other offers for the company are expected to have come in but before shareholders must give a final vote on Snyder's proposal.

The firm also encouraged Six Flags directors to share information promptly with stockholders about bids they receive so that Snyder's proposal can be fairly evaluated.

The recommendation from Proxy Governance comes two days after Rockville-based Institutional Shareholder Services Inc., by far the dominant voice in proxy advising for shareholders, issued a report urging Six Flags shareholders to back Snyder. Proxy Governance is a year old, and it would not discuss how many clients it has or which ones hold Six Flags stock.

Snyder, Six Flags' largest shareholder, wants to remove two executives and one board member and install himself, former ESPN executive Mark Shapiro and local builder Dwight C. Schar into leadership positions. Snyder and Shapiro want to restructure the company's marketing and operations and make its parks more family friendly.

In its report, Proxy Governance echoed many of the criticisms of Six Flags' management made by Snyder and ISS, saying the company had "significantly underperformed" compared with competitors for the past five years. Proxy Governance added that Six Flags' board ignored previous shareholder complaints about the company's performance and said its attempt to boost the stock by putting the company up for sale in August "came too late."

Six Flags officials have cited a recent increase in revenue and attendance as proof that their plan for the company is working. But Proxy Governance was not convinced, concluding "while we acknowledge that the company has shown some financial improvement of late, we support [Snyder's] plan to focus on a more aggressive turnaround."

Snyder, who owns an 11.7 percent stake, needs the support of owners of a majority of shares to succeed. To date, New York investor Simon Glick, who owns about 9.8 percent, is the only major investor to publicly back Snyder.

Snyder has offered to pay $6.50 a share to increase his stake, but only if shareholders agree to turn the company over to his team. Since he launched his bid, however, the stock has traded above $7.

Six Flags officials, who told shareholders they would get a better deal by selling the company, said "numerous" prospective buyers have approached them.

Glass Lewis & Co., a San Francisco-based proxy services firm, is also expected to weigh in on the Snyder-Six Flags battle as early as next week, said spokeswoman Loren Schaffzin.

Daniel M. Snyder is trying to take over Six Flags Inc.