BACKBENCH MPs on both sides of politics opposed to the government's internet filtering proposal are vigorously lobbying their colleagues, creating a potential roadblock to the plan backed by the Communications Minister, Stephen Conroy.

A group of four young Liberal MPs - Simon Birmingham, Alex Hawke, Michael Johnson and Jamie Briggs - are leading the charge against the filter within the Coalition, while the Labor senator Kate Lundy is putting a case to her colleagues in favour of an optional filter.

Senator Conroy has won the backing of cabinet and is awaiting debate about the internet filtering plan in the party room next month.

Senator Conroy has won the backing of cabinet and is awaiting debate in the party room next month, while the Coalition is waiting for more detail. With the Greens indicating their opposition, the Coalition's position is likely to decide the filter's fate.

The government's proposal involves internet service providers blocking access to websites that appear on a blacklist because of content that falls foul of Australia's classification guidelines, including portrayals of sexual violence and instructions on committing crime.