New libel laws aimed at giving better protection to people expressing their opinions are to come into force in England and Wales.
Claimants will have to show they have suffered "serious harm" before suing, under the Defamation Act 2013.
The changes - coming in on 1 January - will also address "libel tourism".
The government said the law would reverse the "chilling effect" current libel laws have had on freedom of expression and legitimate debate.
Journalists, scientists and academics have faced unfair legal threats for fairly criticising a company, person or product in the past, the Ministry of Justice said.
It said the Act would provide "clearer, better protection for people publicly expressing opinions", while campaigners said it was "good news for free speech" but urged Northern Ireland to follow suit.
The Defamation Act 2013 contains a series of measures, including:
- "New serious harm threshold" aimed at helping people to understand when claims should be brought and discourage wasteful use of court time
- Protection for scientists and academics publishing peer-reviewed material in scientific and academic journals
- Protection for those publishing material on a matter of public interest where they reasonably believe that it is in the public interest
- Libel tourism targeted by tightening the test for claims involving those with little connection to England and Wales being brought before the courts
- Introduction of a new process aimed at helping potential victims of defamation online, by resolving the dispute directly with the person who has posted the statement
- Single-publication rule to prevent repeated claims against a publisher about the same material
Justice minister Shailesh Vara said: "As a result of these new laws, anyone expressing views and engaging in public debate can do so in the knowledge that the law offers them stronger protection against unjust and unfair threats of legal action.
"These laws coming into force represent the end of a long and hard-fought battle to ensure a fair balance is struck between the right to freedom of expression and people's ability to protect their reputation."
The Libel Reform Campaign, which is made up of Index on Censorship, English PEN and Sense About Science, has been demanding new legislation since 2009.
Mike Harris, of the campaign, said it was "good news for free speech".
"The Defamation Act was intended by politicians to end the chill from the archaic libel laws of England and Wales," he said.
"It's taken four years, support from 60,000 people and a cross-party consensus to get to where we are today.
"We hope the judiciary will take note, and that in the future open debate on matters in the public interest will not be chilled by litigious oligarchs or corporations."
Tracey Brown, director of Sense About Science, said the Act was a "major step".
But she added: "A lot will depend on how the courts apply the new law. We will keep it under review to see that the law does give scientists the increased confidence to publish that it promises."
'Fit for 21st Century'
The new Act applies only to England and Wales - Scotland has its own law and Justice Minister Kenny MacAskill has said he believes it is "robust enough for present purposes".
The Stormont executive is currently considering whether to bring its libel laws inline with Westminster.
Jo Glanville, director of English PEN, said England and Wales now had laws "fit for the 21st Century but the battle's not over yet".
"It's essential that Northern Ireland now adopts the act. Otherwise the new law could be fatally undermined and cases could be heard in Belfast under the old legislation," she said.
The main political parties committed themselves to reviewing libel law in England and Wales in their manifestos at the last general election.
A draft bill was published in 2011, with the Defamation Bill outlined in the Queen's speech in May last year. The Defamation Act 2013 received Royal Assent on 25 April.