Liberal Democrats are preparing to demand that David Cameron drops his plan for “opt-out” internet porn filters to be installed in homes.

Internet service providers are rolling out network-wide internet censorship controls in time for a government deadline of the end of the year. They will require every household to choose whether or not they want to be able to access legal pornography sites.

But in the latest sign of potential conflict between the Coalition partners, senior Lib Dems are preparing to commit the party to a policy of demanding that the Government drops opt-out filters.

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Tim Farron, the Liberal Democrat President, has drafted a motion for the party’s Spring Conference, decrying the policy which it says would prevent people from accessing legitimate websites while “giving parents a false sense of security”.

It calls on ministers to bring in legislation enshrining the “digital rights of the citizen” which would include stopping “any requirement for opt-ins, opt-outs, filters, lists or controls on legal material”. Internet providers would still be free to offer parental controls or filters, but it would be up to “families and individuals to decide if and how they wish to use them”, it adds.

Speaking to The Independent, Mr Farron said there was growing evidence that the filters were ineffectual and doing more harm than good.

“Essential sites on sexual health, gender and sexuality, domestic violence and LGBT rights are being blocked by these filters, whilst pornographic content is still available,” he said. “Our motion is designed to strengthen Lib Dem ministers’ hands in challenging this nonsensical policy, which has yet to be brought before the House of Commons.”

Mr Farron described the plans as “misconceived, ineffective and illiberal” and said they were designed to win Mr Cameron headlines, instead of protecting children.

“If the Prime Minister really wanted to protect children from inappropriate material, he’d ensure they had access to good sexual health and relationship education and give parents the help and support they need to talk to their children about this issue,” he said.

Sources close to the Liberal Democrat leader Nick Clegg said the Deputy Prime Minister would not attempt to prevent the motion from being adopted when it is debated in March. Liberal Democrats have a strong libertarian tradition – and selected Anna Arrowsmith, the first woman in Britain to direct adult movies, as a candidate in the 2010 General Election.

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Party sources said they had been inundated with complaints from members who had signed up to the new filters – only to find legitimate sites including the Lib Dems own LGBT section blocked.

This reflects independent research that found some of the filters initially blocked sites including ChildLine, a domestic abuse helpline and advice sites for sexually transmitted diseases. At the same time, web developers have created an add-on for the popular Google Chrome browser which allows users to override the filters imposed by internet providers. It is called “Go Away Cameron”.

But the Tories believe that the party’s tough line on internet porn will play well electorally and appeal to parents worried about the proliferation of easy to access hardcore images and videos.

Party sources described the new Lib Dem approach as “disappointing”.

“Tim Farron clearly does not want to prioritise the safety of our children online or support our efforts to prevent anyone accidentally accessing illegal material,” they said.

Controls: The unintended consequences

Sexual health information

A BBC survey found that TalkTalk’s filter failed to block 7 per cent of the 68 pornographic websites tested – but labelled the Edinburgh Women’s Rape and Sexual Abuse Centre website “pornographic”.

Charity websites

O2 was forced to amend its “porn filter” after it began inadvertently blocking access to charity websites, which included ChildLine, the NSPCC and the Samaritans. The ‘opt in’ filters also denied access to Parliament and government websites.

...and the easy way around it

Meanwhile, a Singapore-based computer science graduate has created a new browser app, cheekily called “Go Away Cameron”, that allows users to browse any site they wish, even if the parental controls are switched on, by directing requests through a so-called “proxy” server.