England's libel law is being used to bully people into silence, quash dissent and destroy criticism, comic Dara O'Briain said today.
O'Briain backed the need for urgent reform of a "ridiculous system" that was attracting libel tourists and threatening freedom of speech.
The performer and author was speaking the day before leading academics, medical and science editors and human rights activists will lobby MPs and peers to reform libel laws.
Last month, Justice Secretary Jack Straw said he was preparing proposals for reform, calling the current system "unbalanced".
Speaking at the launch of the campaign at The Law Society in central London, O'Briain said: "The libel laws which were initially set up to protect the reputation of individuals at a time when companies weren't the entities they are now are being used by companies to essentially quash dissent and to destroy criticism.
"That's a major problem. Companies can basically bully people out of saying bad things about their products and services."
He added that the "ridiculous" system meant discussion of issues in the public interest, such as dubious medical practices or the benefits of a new pill, were being prevented by the fear of libel.
The Mock The Week presenter also joked his support came at "great personal cost" as he had run a travel agency specialising in libel tourism for many years.
Roger Highfield, New Scientist editor, agreed the libel laws were having a "chilling effect on the discussion of medical therapies".
Author AC Grayling said it was a "profound injustice" that big organisations could "buy the silence of other people".
The libel laws were "an instrument to bash other people with and that's just unacceptable", he said.
Broadcaster Nick Ross added the laws amounted to "bullying done under the veneer of respectability and decency" and libel was a "shameful process".
"The law is simply on the side of the devil, as far as I can see," he said.
Stand-up comic Alexei Sayle, who won a libel case brought against him, added it was "absurd" that he could have lost everything by "saying some words", and joked that he may as well have "stabbed the f*****".
Tracey Brown, managing director of Sense About Science, one of the organisations calling for reform, said the fear of libel was "so damaging".
Human rights groups are holding back on criticising companies, critical biographies were being hindered, and US newspapers were considering blocking users with UK ISP addresses, she said.
Ms Brown added there was "international embarrassment" over the laws, which were also causing medical journals to hold back publication of reports, blocking critical reviews of misleading therapies and leading to fraudulent scientific papers not being withdrawn.
The Coalition for Libel Reform, backed by English PEN, the Index on Censorship, Sense About Science and Reporters Without Borders, said the cost of a libel trial was often in excess of £1 million and 140 times more expensive than libel cases in mainland Europe.
Stephen Fry, who was not at today's launch, said the current situation with "archaic, unfair and illiberal libel laws" was making England a "global laughing stock".
Last month, Mr Straw said radical change was needed.
Media outlets and individuals had to be given clearer rights to freedom of expression, he said.
Freedom of speech campaigners have suggested that libel payouts could be capped at £10,000, and apologies made the main remedy.
There have also been calls for the burden of proof to be shifted, so claimants have to demonstrate damage.