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Britain looks set to re-open the question of how adult internet content is regulated, as embattled Prime Minister David Cameron scrambles for ways to shore up support from the conservative heartland.

According to a report in The Times, the U.K. government is planning to a review of current rules to consider whether explicit content could be better screened from children — and is pushing the possibility that households will have to opt in with their ISP if they don’t want all adult content to be blocked:

Cameron will reveal that the Government is to consult on whether adults wanting to view porn should be forced to “opt in” in order to access porn sites, a measure fiercely resisted by some internet service providers (ISP) and anti-censorship campaigners. The decision to explore the option of blocking porn by default came after a Downing Street breakfast meeting with creative-industry figures yesterday. “We will consult on a default option,” a Whitehall source said. “Nothing is ruled in or out at the moment. We will look at all the options.”

Next on his list for a meeting are internet service providers, who are likely to voice concerns about the prospect of a mandatory block.

The question of network-level blocking of adult content (primarily pornography) is a perennial topic of debate in censorious, prudish Britain. The argument is that children can too easily find their way to explicit content on the web, and in order to save clueless parents the default position should be that controls are put in place.

It’s a subject that has come up regularly over the past few years, including a push in 2010 soon after Cameron took office. In fact, last year Britain’s four biggest internet providers signed a voluntary code that meant they would offer customers the chance to put a block on adult content. But it is applied in a variety of ways, and only applies to new subscribers: not enough, apparently, to quell the outcry.

Protecting children online is always a hot topic — only yesterday we reported about European-level plans to look at age verification on the web — but there is a bigger political context here worth remembering.

Cameron has endured a troubled time recently: he’s being hammered over a problematic budget, with spreading anger that his government’s austerity measures seems to hurt ordinary people but protect those with high incomes.

That led to a backlash on Thursday in local elections, with his Conservative party losing control of a large chunk of the cities and authorities it had previously commanded. Plus, of course, there’s the ongoing question of the relationship between Downing Street and the Murdoch empire, with inquiries into the actions of News Corp’s (s NWS) British newspaper businesses turning the heat up on Cameron thanks to his close links to the mogul.

At the same time, Cameron’s political rivals have made hay from his close connections to Google (s GOOG), claiming that his relationship to the search giant is one reason he has been lenient on internet porn (despite the fact, as we’ve shown, he’s not.)

No doubt emboldened by the recent court ruling that forces British ISPs to block The Pirate Bay, the Conservative right see an easy win with heartland voters by targeting adult-only content. Although it never quite tackles the technical problem of identifying explicit content, it’s clearly seen as a valuable rhetorical tool.

It’s clear where the support for this move comes from: now it will be interesting to see how much resistance it gathers, and from where.