David Cameron has announced "radical" plans to help parents protect their children online by prompting them to tailor their computer settings to block internet pornography, violence and other unsuitable websites.

The prime minister, who described such material as "a silent attack on innocence", said the proposals were intended to safeguard childhood and make sure it was "untainted by the worries and complexities of adulthood".

Under the plan, people switching on a new computer will be asked whether there are children in the house. If there are, parents will then be prompted to tailor their internet filters. The technology will also show parents how to restrict access to social media sites after a time of their choosing, to narrow the number of sites that younger children can visit, and to block access to certain sites altogether.

As backup, said the prime minister, the filters against the most obvious threats – such as pornography and self-harm sites – would remain on if parents repeatedly clicked OK to get through the filter set-up quickly. Providers would also have to verify the age of the person setting the controls so that children could not set up the filters.

Writing in the Daily Mail, Cameron said the government had opted for the system because the evidence suggested that having "default on" filters as standard on all new computers was a waste of time.

The blanket-filter system, he said, was so restrictive that parents had found they were unable to access such services as on-demand TV and had simply turned the filters off because they were too annoying.

"The point is we need a more sophisticated system than this – one that allows parents to tailor exactly what their children can see," said Cameron. "This is what child safety experts recommend."

He added: "With our new system, every parent will be prompted to protect their child online. If they don't make choices, protection will be automatically on. No other government has taken such radical steps before. And once all this is in place, Britain will have the most robust internet child protection measures of any country in the world – bar none."

The PM said he had appointed the MP Claire Perry – "a passionate campaigner for internet safety and mother of three" – as his adviser on preventing the sexualisation and commercialisation of childhood, and to see the proposals through and get service providers involved.

"All this comes back to something really important," he said. "It's not just about the internet, or modern technology, it's about childhood.

"These should be distinct and precious years, full of security and love, untainted by the worries and complexities of adulthood. From one parent to another, I pledge to do whatever I can to preserve that innocence and protect our children. Nothing matters more."