The government says its new internet laws will help keep children safe and combat racism and other abuse.
The draft Online Safety Bill includes:
* Ofcom, the new online regulator, being able to fine companies up to £18m or 10% of their annual global turnover (whichever is higher) if they fail to take down harmful content
* Ofcom having the power to block access to sites
* New duty of care rules requiring tech companies to take action not only against dangerous content but also against content that is lawful but harmful, such as information about suicide and self-harm
* The threat of criminal action against senior managers if tech companies fail to live up to their responsibilities, with the new rules being reviewed every two years
* Tech firms will have to take responsibility for fraudulent user-generated content, including financial fraud such as romance scams or fake investment opportunities
* The protection of "democratic content", meaning platforms will not be able to discriminate against political viewpoints and otherwise banned content will be allowed if it is "democratically important"
The pressure has been growing on the government for years to act against online abuse.
The issue was brought into focus after a large group of sports, athletes and organisations recently took part in a social media boycott, protesting against the lack of action taken against online abuse.
Digital Secretary Oliver Dowden said the "ground-breaking laws" would "usher in a new age of accountability for tech and bring fairness and accountability to the online world".
"We will protect children on the internet, crack down on racist abuse on social media and through new measures to safeguard our liberties, create a truly democratic digital age," he said.
Home Secretary Priti Patel added: "It's time for tech companies to be held to account and to protect the British people from harm. If they fail to do so, they will face penalties."
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Not everyone thinks the new rules are enough, however, with the NSPCC warning that they fail to offer the comprehensive protection that children need on social media.
Sir Peter Wanless, chief executive of the children's charity, said: "Government has the opportunity to deliver a transformative online safety bill if they choose to make it work for children and families, not just what's palatable to tech firms.
"The ambition to achieve safety by design is the right one. But this landmark piece of legislation risks falling short if Oliver Dowden does not tackle the complexities of online abuse and fails to learn the lessons from other regulated sectors."
Labour's shadow culture secretary Jo Stevens said: "There is little to incentivise companies to prevent their platforms from being used for harmful practices.
"The bill, which will have taken the government more than five years from its first promise to act to be published, is a wasted opportunity to put into place future proofed legislation to provide an effective and all-encompassing regulatory framework to keep people safe online."