A plan by the the fifth largest mobile carrier in the United States to block access to certain web content based on its customers' data plan could be a violation of new "net neutrality" regulations.

MetroPCS has announced it will be offering three different pay-as-you-go mobile data plans that will make different parts of the web available depending on how much the customer pays.

The lowest plan, starting at $40 per month, offers unlimited talk, texting, web browsing, and access to YouTube, but blocks other streaming video sites, like Vimeo. The use of internet phone calling apps like Skype and the internet radio Pandora are also expected to be blocked.

For an extra $10, MetroPCS customers will be able to have access to an additional 1 GB of data as well as the ability to download music when connected to a Wi-Fi network. The company's $60 per month plan provides customers with unlimited data access and MetroSTUDIO premium content such as video-on-demand channels and audio downloads.

The three tiered data plan may violate FCC "net neutrality" rules passed in December because it restricts what services and sites are available to its users. The media advocacy organization Free Press is urging the FCC to investigate the mobile carrier for building a "walled garden for the mobile web."

"In December, the FCC chose to disregard wireless protections in its Net Neutrality order, and MetroPCS’s new scheme is a preview of the wireless future in a world without protections on the mobile Web," Free Press Policy Counsel M. Chris Riley said. "Such blocking of websites, services or applications would clearly be prohibited and deemed unreasonable on a cable or DSL network. Are these the kinds of restrictions the FCC really wants to promote on wireless networks?"

The new "net neutrality" regulations do not prevent companies from selling bandwidth in capped tiers, with overage charges for users who download more than their permitted share of information, but the regulations do restrict "unreasonable discrimination" against web content, such as the outright blocking of certain sites or software services.

It is unclear what the FCC considers "reasonable" discrimination and it is likely to make such decisions on a case-by-case basis.

Tiered pricing structures are already in place for many communications providers like AT&T and Cricket, which offer wireless broadband services. Verizon said it would implement similar pricing structures in the coming months.

"The open Internet order approved in December stated that the FCC was not implicitly approving practices on the mobile Web that violate its rule against unreasonable discrimination – and now we’ll see whether the agency is willing to do anything about such practices," Riley continued. "Silence in the face of ongoing violations is no different from outright approval."

Roger D. Linquist, president, CEO and chairman of MetroPCS claims the three tiered data plan allows for "more video, more sharing of their content and more Web browsing capabilities" and lets consumers decide how much additional data access they want to pay for.

"MetroPCS’s plan will restrict consumer choice and innovation in a developing mobile market, all for the sake of further padding its bottom line," Riley added. "The FCC must not stand idly by while carriers are engaging in anti-consumer and anti-competitive behavior, and we urge the agency to investigate."