At a public meeting, the FCC unanimously approved an inquiry exploring how companies that provide high-speed Internet service manage Web traffic, and whether consumers of such broadband service are adversely affected.

Neutrality supporters, including such Internet content firms as Google and Yahoo, insist safeguards are needed to prevent Internet service providers from acting as content gatekeepers.

But critics, including leading providers of broadband service like Verizon Communications and AT&T, say such regulation would deter investment and is unnecessary.

The FCC acted amid growing criticism of Chairman Kevin Martin, a Republican, by the newly empowered Democratic congressional majority -- some of whom have been critical of Martin's deregulatory agenda and what they perceive as his heavy-handed style.

The Senate Commerce and the House Energy and Commerce panels already have held two FCC oversight hearings, with more planned.

Dissension over Internet regulation engulfed last year's congressional debate on telecommunications law and contributed to the demise of major Senate legislation.

Disagreement over the issue also was apparent among FCC members, who appeared split along party lines.

Martin, who has repeatedly expressed skepticism about network neutrality, insisted the FCC has been vigilant in responding to concerns about anti-competitive behavior, and that the commission has shown a willingness to act and is committed to hearing all sides.

But one of the agency's two Democratic members, Michael Copps, was outspoken in advocating stringent safeguards, declaring, "I want an FCC that unconditionally states its preference for non-discrimination on the Internet."

Both Copps and the commission's other Democrat, Jonathan Adelstein, complained that an order of inquiry lacks the teeth of a rulemaking, which can result in policy changes. Adelstein said the vote in favor of an inquiry sends a message of "how low this ranks."

Republican Commissioner Robert McDowell, however, said while it is essential for the FCC to gather data, "We must also resist the temptation to impose regulations that are based mainly on theory."

Consumer groups expressed unhappiness with Thursday's action. "This bureaucratic process will delay [action] by months if not years" due to the lack of a rulemaking, a spokesman for the public advocacy group Public Knowledge said.

But AT&T and Verizon applauded the FCC move, predicting the evidence to be gathered would highlight how broadband players benefit consumers.