WASHINGTON, June 30 (Reuters) - Asset managers who use proxy advisory firms to help them make corporate voting decisions on behalf of investors should be routinely reviewing their policies to ensure they are acting in their clients' best interest, U.S. regulators warned on Monday.

The warning by the U.S. Securities and Exchange Commission was posted late Monday evening as part of a new legal bulletin outlining guidance, both for investment advisors that use proxy advisory firms and for the proxy firms themselves.

The guidance was issued in response to growing concerns in some circles about the role that proxy advisory firms like Glass Lewis and Institutional Shareholder Services play in corporate elections.

Critics like the U.S. Chamber of Commerce have said that proxy advisory firms wield too much influence in corporate elections and may pose conflicts of interest that are not always well-disclosed.

But mutual funds, pension funds and other institutional investors who often hire proxy firms to help them decide how to cast votes on critical matters such as executive compensation, have said the firms provide a valuable service - especially for smaller funds who lack enough staff to parse through voting decisions for thousands of companies.

Although the chamber has long been calling for reforms, the issue started to heat up last year when SEC Republican Commissioner Daniel Gallagher gave a speech lambasting the role that proxy advisory firms play in corporate elections.

In particular, Gallagher took aim at prior SEC rules and other guidance which he said enabled investment advisers to overly rely on proxy advisory firms for voting recommendations.

This, he said, clashed with federal laws which impose a fiduciary duty on advisers to act in their clients' best interest.

Since that speech, the SEC's other Republican Commissioner Michael Piwowar has also called for changes. In response to the concerns, the SEC held a roundtable on the subject late last year, and SEC Chair Mary Jo White said in a March speech that guidance would be issued to address some of the concerns.

That guidance on Monday lays out exactly what responsibilities investment advisers have when using proxy advisors. They must, for instance, periodically sample proxy votes to ensure compliance and review the adequacy of their proxy voting policies once a year.

In addition, the guidance also lays out steps that proxy advisory firms should follow, including when they may need to disclose potential conflicts of interest.

If, for instance, a proxy firm provides consulting services to a company on a matter at the heart of a voting recommendation, it must be disclosed if it is deemed a "material" interest.

A copy of the guidance can be found on the SEC's website at http://www.sec.gov/interps/legal.shtml (Editing by Stephen Coates)