Online companies could face fines or prosecution if they fail to remove illegal content, under Conservative plans for stricter internet regulation.
The party has also proposed an industry-wide levy, dubbed a “Twitter tax”, to fund “preventative activity to counter internet harms”.
Labour said it had “pressed for tough new codes” in the past but the government had “categorically refused”.
The Liberal Democrats said more needed to be done “to find a real solution”.
The Conservatives said the levy, proposed in their election manifesto, would use the same model as that used in the gambling industry, where companies voluntarily contribute to the charity GambleAware to help pay for education, research, and treating gambling addiction.
All social media and communications service providers would be given a set period to come up with plans to fund and promote efforts “to counter internet harms”.
If they failed to do so, the government would have the power to impose an industry-wide toll.
The Conservatives say the exact details, including how long the industry will be given to comply and the size of the levy, will be consulted upon.
A Labour spokesman said: “If the Tories are planning to levy a new tax on social media companies, they need to set out how it will work, who it will affect and what it will raise.
“Labour has pushed for a code of practice about the responsibilities of social media companies to protect children and young people from abuse and bullying.”
The Conservatives have also pledged to introduce “a sanctions regime” that would give regulators “the ability to fine or prosecute those companies that fail in their legal duties, and to order the removal of content where it clearly breaches UK law”.
Social media platforms and internet service providers would have clearer responsibilities regarding the reporting and removal of harmful material, including bullying, inappropriate or illegal content, and would have to take down material.
“It is certainly bold of the Conservatives to boast that they can protect people on the internet,” Liberal Democrat home affairs spokesman Alistair Carmichael said.
“Government and technology companies must do more to find a real solution to problematic content online.”
And Labour’s digital economy spokeswoman Louise Haigh said: “The Home Office were crystal clear they did not want to legislate and that they believed the voluntary framework was sufficient.
“The fact is that in government the Tories have been too afraid to stand up to the social media giants and keep the public safe from illegal and extremist content.”