A fifth of children aged five to seven are surfing the internet without an adult in the room, new research shows.
The news will raise concerns about access to adult material and grooming by paedophiles.
Despite parents’ fears, less than half of those whose children use the internet at home have controls in place to stop access to adult material.
Websites such as Nickelodeon Kids, Disney and CBBC are blamed for encouraging ever younger children to use the internet.
Youngsters are being flooded with advertising for toys and foods and sites such as YouTube and Twitter are accused of exposing young children to adult material.
The media regulator Ofcom, which conducted the research, said: “There has been an increase in the number of younger children using the internet at home.
“This increase does not appear to be the result of increased internet take-up within the home, suggesting that children in this age group are starting to use the internet at home at a younger age.”
The editor of New Media Age magazine, Justin Pearse, said it is “shocking” that so few parents are taking steps to protect youngsters.
He said: “The internet industry has done a huge amount to help parents monitor and control children’s web access. However, there is still a huge amount of dangerous and disturbing content that can be accessed by a child as young as five”.
“There is very good software available, consequently it is quite shocking to see research that less than half of parents are making use of it.
“Children are increasingly living out their lives via the web and accessing it in their own rooms via laptops where there is no parental supervision. That leaves them open to adult material.”
Mr Pearse criticised websites such as Facebook and Twitter, which he said carry a lot of age sensitive material, but neither has any sort of enforceable age controls in place.
Ofcom has produced a list of the 50 most popular websites visited by children and the top ten includes Facebook, YouTube and Disney.
There is growing concern that young children are also accessing pornography via internet on their mobile phones, and the new research shows that there is an increasing number of children aged five to seven who now own a personal mobile phone.
Last month the National Association of Head Teachers said that mobile phone pornography in schools is now “widespread and very worrying”.
In August The Guardian columnist Oliver James warned that the younger a person starts watching pornographic material, the earlier they have sex.
Mr James was writing in response to research which concluded that the ease with which hardcore pornography can be accessed online means almost all 14 to 17-year-olds will be exposed to it at some point.