Pornography websites must adopt age-restriction controls or face closure, the Conservatives say.

Culture Secretary Sajid Javid said the party would ensure under-18s were prevented from seeing adult content.

In a recent Childline poll, a tenth of 12 to 13-year-olds were worried they were addicted to pornography.

The plan is targeted at both UK-based and overseas websites and would be introduced if the Tories are in power after the general election.

Experts have welcomed the idea but warned it would be difficult to implement.

Media playback is unsupported on your device Media caption Culture Secretary Sajid Javid: New restrictions key for "protecting children"

The proposed system would be overseen by an independent regulator with the power to force internet service providers to block sites which failed to include effective age verification.

Shared links

Providers who did not co-operate could also be fined.

Mr Javid said: "If you want to buy a hardcore pornography DVD in a store you need to prove your age to the retailers.

The number of ChildLine counselling sessions regarding porn more than doubled last year to over 1,100 Peter Wanless, NSPCC chief

"With the shift to online, children can access adult content on websites without restriction, intentionally or otherwise.

"That is why we need effective controls online that apply to UK and overseas. This is about giving children the best start in life.

"We do not want to prevent adults from accessing legal content but we do want to protect our children from harmful material, so they are free to develop a healthy attitude to sex and relationships."

He told BBC Breakfast an effective age control mechanism might be the use of credit cards.

'Deepening concern'

He said in some countries, such as Finland, there are EIDs - electronic IDs - in place.

"So there are ways there and there might be new ways in the future. The key thing is, it's not for me or politicians to decide what's effective. That will be the job of a regulator, but it is possible," he said.

Sarah Green, acting director of the End Violence Against Women Coalition, said: "Surveys have shown that more than half of young people have seen online pornography by the age of 14, and that many see it without even seeking it out as links are shared on social networks."

NSPCC chief Peter Wanless said the easy availability to children of online pornography was of "deepening concern".

He added: "It can leave them feeling frightened, confused, depressed or upset. The number of ChildLine counselling sessions regarding porn more than doubled last year to over 1,100 with some young girls revealing they were being pressured to mimic scenes from adult films.

"Any action that makes it more difficult for young people to find this material is to be welcomed.

"The key will be making any system work effectively as we know foreign outlets are recklessly flooding the market with films that can be viewed by anyone without any age checks in place."