More than half of free wi-fi hotspots in the UK allow unfiltered access to adult content, prompting calls for tighter restrictions.
Researchers went into 179 cafes, restaurants, hotels, retailers and public spaces in Birmingham, Manchester and London, and attempted to log on to websites that are legal, but have explicit content.
The study found 30% of cafes and restaurants have no filtering in place to prevent access to pornographic sites, while 20% fail to restrict customers from accessing online sex dating websites.
In addition 53% do not restrict online stores selling knives and swords, and 80% granted full access to drug-related content.
Graeme Coffey from mobile security firm AdaptiveMobile, who carried out the research, said: "For every parent across the UK this report will come as an unwelcome surprise.
"In the last two years there have been two convergent trends: a big increase in public wi-fi or 'hospitality wi-fi' and greater access to smartphones, gaming consoles and tablets with a wi-fi capability, the kind of device a child could have.
"Most people will instinctively block adult content when it comes to filtering, but what these results show is that we should also be looking at content related to drugs and violence which are just as harmful but frequently overlooked."
Seamus Kelly, the father of two teenagers, was surprised to find out the cafe he visits in Manchester's Northern Quarter has unrestricted internet access.
He told Sky News: "I think in some ways the biggest issue is not knowing what they're looking at.
"If it's at home you might have a certain amount of control over the system, but when it's out not only do you not have control but you've also got absolutely no idea what they may or may not be coming across and it is quite worrying."
Outside a cafe in south Manchester a group of 15 and 16-year-olds told Sky News they have all had smartphones from the age of 13 and frequently use wi-fi to go online.
They admitted they know people who use sites their parents would not allow them access to.
Earlier this year David Cameron backed a campaign by children's charities calling for a ban on adult content on all public wi-fi.
John Carr is among the world's leading authorities on children's and young people's use of the internet.
He told Sky News: "Virgin, O2 and Sky already block access to adult content and the other major wi-fi providers in the UK - BT, Arqiva and Nomad - have announced their intention to do the same by the end of this year.
"There is also some talk about developing a 'kitemark' to allow shops, hotels, trains, buses that provide wi-fi access to confirm that they provide a filtered service."
A Department of Culture, Media and Sport spokesperson said: "The Government is committed to protecting children online which is why we have been working closely with the major Internet Service Providers, who have already put filters in place anywhere children are likely to be to block pornographic content.
"The main providers cover 90% of all internet connections in the UK and this report shows that our approach is working and we are ahead of many other countries in protecting children online."