Subscribers to four of the UK's biggest internet service providers will have to "opt in" if they want to view sexually explicit websites, as part of government-sponsored curbs on online pornography.

The measures will be unveiled on Tuesday as David Cameron hosts No 10 meeting with the Mothers' Union, which earlier this year produced a raft of proposals to shield children from sexualised imagery.

The prime minister is expected to announce other moves in line with the Christian charity's review, such as restrictions on aggressive advertising campaigns and certain types of images on billboards.

There will also be a website, Parentport, which parents can use to complain about television programmes, advertisements, products or services which they believe are inappropriate for children.

The site, which will direct complaints to the regulator dealing with that specific area of concern, is expected to be run by watchdogs including the Advertising Standards Authority, BBC Trust, British Board of Film Classification, Ofcom, Press Complaints Commission, Video Standards Council and Pan European Game Information.

The service providers involved are BT, Sky, TalkTalk and Virgin. Customers who do not opt in to adult content will be unableto access pornographic websites.

Cameron gave strong backing in June to the Mothers' Union proposals after he commissioned a six-month review by the charity's chief executive, Reg Bailey. However, Cameron did not commit to legislation.

Bailey's recommendations included providing parents with one single website to make it easier to complain about any programme, advert, product or service, putting age restrictions on music videos and ensuring retailers offer age-appropriate clothes for children.

Cameron wrote to Bailey in June to thank him for his report. "I very much agree with the central approach you set out," the letter said.

"As you say, we should not try and wrap children up in cotton wool or simply throw our hands up and accept the world as it is. Instead, we should look to put 'the brakes on an unthinking drift towards ever-greater commercialisation and sexualisation'."

Bailey's report asked for government and business to work together on initiatives such as ending the sale of inappropriately "sexy" clothing for young children, for example underwired bras and T-shirts with suggestive slogans.

However, he recommended that if retailers do not make progress on the issue they should be forced to make the changes in 18 months.