Almost half of internet users in the UK have suffered "harm" online, suggests research.
The harms include bullying and harassment as well as attacks by malware or cyber-thieves.
The study by regulator Ofcom sought to quantify the type and severity of harm that going online could do to people.
Interactions via social networks and email were deemed to be the most likely to cause harm.
The study questioned almost 2,000 Britons and found that 45% reported they had suffered some form of harm online.
Ofcom looked at harm in four areas:
- online content that people see, hear or watch
- interactions with other users
- data and privacy
- cyber-security and hacking
Bad experiences included unwelcome attention via social media, trolling and bullying as well as theft of data or personal information.
Across all these categories, roughly 20% of people reported that their experience had been "very harmful".
About 20% of those questioned said they reported offensive or harmful content when they encountered it. Hate speech, harassment and illegal sexual content were most likely to be reported.
In addition, many of those questioned were concerned about the effect on children of easy access to the net and potentially harmful content or interaction.
The research has been released shortly before Ofcom boss Sharon White gives a speech at the Royal Television Society conference in which she is expected to say the harms have arisen because of a lack of regulation.
"While the regulation of online content has evolved in recent years, there are significant disparities in whether and how it is regulated," she said in a statement released alongside the research.
Ms White suggested that some of the principles underlying what the UK's broadcasters can show could help shape regulation for social media and other online platforms.
The final form of regulation should be up to government and Parliament, said Ms White in the Ofcom statement. But she said looking at the way broadcasters handle freedom of expression, transparency and how the enforcement of regulations is handled could guide debate.
If fines are chosen as a punishment, online firms should also be subject to "meaningful financial penalties" if they flout rules or fail to protect users, said Ms White.
The call was echoed by Liam Byrne, Labour's shadow digital minister, who said net firms should "step up to their responsibilities".
The study comes as social media firms face increasing scrutiny over how they handle potentially harmful or inappropriate content.
Earlier this month, Jean-Claude Juncker, president of the European Commission, said Google, Facebook and Twitter must remove extremist content within an hour or face hefty fines.
In his annual State of the Union address to the European Parliament, he said an hour was a "decisive time window".