Yahoo and AOL UK are among the signatories to an open letter to David Cameron calling for reform of UK legislation to better protect online publishers, blogs and forums.

Parenting forum Mumsnet and industry group the Internet Service Providers' Association (ISPA) have also signed the letter, below, which says that libel threats are curbing free speech online.

Emma Ascroft, director of public and social policy for Yahoo UK & Ireland, Lisa Fitzgerald, senior counsel for AOL UK, Nicholas Lansman, secretary general for the ISPA and Justine Roberts, CEO of Mumsnet have all put their names to the letter. In it they ask the prime minister to update existing libel legislation and provide protection for online discussion in the government's draft libel bill.

The letter is part of the libel reform campaign led by Sense About Science, English PEN and Index on Censorship, which launched its proposals to reform UK libel legislation a year ago. Hopes of a new Defamation Bill were strengthened over the summer by Lord Lester's draft Defamation Bill and work on legislative reform, which included the discussion of forums such as Mumsnet as 'innocent facilitators' in online discussions and not liable for defamation. The government will now produce its own bill by the 2011/12 parliamentary session.

"The English law of defamation is having a disproportionate, chilling effect on online writers, e-communities and web hosts: the libel laws have not been updated to address the rise of online publication. The current multiple publication rule, dating back to 1849, defines every download as a publication and a potential new cause of action," says the letter, which can be read in full below.

The campaign groups are asking for more protection for ISPs, which can currently be held liable for comments they host and for forums and blogs used by "ordinary citizens" to publish. According to campaigners, ISPs have no current defence to an action for defamation under existing libel legislation in the UK.

"The internet has increased everyone's ability to discuss issues such as local politics, medical treatments or the behaviour of institutions. We cannot expect, and the public do not credit, forum discussions with the same standards of fact-checking as national news outlets, but the law currently insists on this. We agree with the service providers that better protection for online discussion is needed in the Government's new Defamation Bill," says Tracey Brown, managing director of Sense About Science, in a release.

The letter sets out four points to be included in the government's draft bill, including the introduction of a single publication rule and a limitation imposed of one year after original publication in which libel actions on internet material can be brought.

"It is essential that libel laws are modernised to allow for the free exchange of information and discussion online. The revolution in technology over the past decade has redefined the very meaning of publishing and the law has not even begun to catch up. As a result, bloggers, ISPs and anyone who posts online are especially vulnerable to threats of legal action. As well as limiting the duration of liability for online publication, new legislation is urgently needed to differentiate between the different modes of online communication and to take account of context. The current lack of distinction only serves to stifle free speech at a time when we should all be enjoying the possibilities of new technology to the full," says Jo Glanville, editor of Index on Censorship, in a release.

The full letter:

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