Backer Marco Rubio said the Senate "should take more time to address the concerns"

Eight US lawmakers have withdrawn their backing from anti-piracy laws, after "blackout" protests on thousands of internet sites.

Two of the bill's co-sponsors, Marco Rubio from Florida and Roy Blunt from Missouri, are among those backing away.

Online encyclopaedia Wikipedia and blog service WordPress are among the highest profile sites to block their content.

The Motion Picture Association of America (MPAA) has branded the protests as "irresponsible" and a "stunt".

The MPAA, Hollywood's primary advocate in Washington and a key supporter of the legislation, is led by former Connecticut Democratic Senator Chris Dodd.

Both bills focus on responding to online piracy, specifically illegal copies of films and other media.

The bills would also outlaw sites from containing information about how to access blocked sites.

The BBC's Jonny Dymond says that with Mr Rubio and Mr Blunt withdrawing their support, the Senate bill - Protect Intellectual Property Act (Pipa) - that had looked likely to pass, now appears to be in trouble.

Mr Rubio is a rising star in the Republican party, and is often suggested as a viable vice-presidential choice for this year's Republican presidential nominee.

Bi-partisan backlash

Republicans and Democrats were among the lawmakers rowing back on Wednesday.

The list of senators no longer backing Pipa includes Mr Rubio and Mr Blunt, and Utah Senator Orrin Hatch, all Republicans, as well as Democrat Ben Cardin of Maryland.

In the House of Representatives, Republicans Ben Quayle of Arizona, Lee Terry of Nebraska and Dennis Ross of Florida said they were no longer supporting the Stop Online Piracy Act (Sopa), joining Pennsylvania Democrat Tim Holden.

Mr Ross tweeted that he was no longer supporting Sopa, because as "a true free marketer, I want IP protected correctly".

In a Facebook posting, Mr Rubio said he and fellow Senators "heard legitimate concerns about the impact the bill could have on access to the internet".

Mr Hatch called Pipa "not ready for prime-time" and said he would remove himself from the bill's list of sponsors.