Care homes scandal: *Gulp* How tweeting can damage your wealth

Perhaps she should have known better, by now. But two weeks after landing herself in trouble with Lord McAlpine with a "mischievous" tweet alluding to the incorrect paedophilia allegations about the Tory peer, Sally Bercow was at it again on Friday.

At about 11pm, the Labour-supporting wife of the Speaker tweeted something about Lord McAlpine's solicitors before clearly thinking better of it and deleting it. In a second tweet which remained visible to her 56,000 followers, she risked fuelling the row between her and the Tory peer by saying: "This is totally politically motivated, I tell you. And I don't do conspiracy theories as a rule."

For someone who, by her own admission, says her husband, John Bercow, takes a lot of flak on her behalf, Mrs Bercow should have been more circumspect, especially as she had reportedly received a letter from Lord McAlpine's solicitors. When alerted by a journalist on Twitter that a letter was on its way, she tweeted: "*gulps*" Her deleted tweet shows the minefield that Twitter users can step into without a moment's thought.

Many of the UK's 10 million-plus tweeters, as well as Facebook users and bloggers, may have had no idea, until now, that they could face the full force of libel law for their tweets, perhaps thinking that the internet carries immunity from defamation. Some Twitter users state on their profiles that retweets do not equal endorsements. Those who work for major companies or the public sector often state their tweets are "my own views". But these caveats do not provide any legal protection for the user themselves. And, as Andrew Reid, Lord McAlpine's solicitor, warned last week, even those retweeting something defamatory about his client could be liable.

In the Commons last week, the Justice Secretary, Chris Grayling, was clear: "It is utterly wrong that anybody should have their name blackened inappropriately and falsely on any form of social media. Of course, the laws of libel apply equally to what is published on a Facebook or Twitter page, as they do to what appears in printed form, so those who are damaged in that way have full legal redress to try and get proper justice done."

But is pointing out a libel as defamatory as the original publication? In 1894, a century before the internet was invented, a Mr Wood was found guilty of libel in the Court of Appeal after he stood silently on a stool on a roadside pointing to a nearby sign which contained defamatory remarks about a Mr Hird. The court ruled that, because Mr Wood had drawn attention to the sign to passers-by, he was technically helping to publish the libel by pointing it out.

When Mrs Bercow tweeted: "Why is Lord McAlpine trending? *innocent face*" she was pointing at the sidebar of trending topics, which included Lord McAlpine's name but didn't point to a defamatory statement. She insists her tweet, which was retweeted 146 times, was not libellous but "mischievous". The words of her tweet may not have been defamatory, but legal sources say that, by pointing out his name, she set off a chain of inquiry which would have directed others to look up the incorrect allegations on Google.

However, in 2009, Google successfully defended itself in the UK against legal action after the High Court ruled it could not be liable for defamatory statements in its search results. The case of MIS Limited vs Google heard that the internet giant "could not prevent the snippet [of defamatory material] appearing in response to the user's request unless it has taken some positive step in advance". Mrs Bercow's tweet, for which she has apologised, clearly could have been prevented by her, but her legal defence could be helped by the Google case.

Despite the High Court ruling, Lord McAlpine's legal team is, The Times reported yesterday, also considering pursuing Google for hosting links that repeat the untrue allegations of abuse by the Tory peer.

Twitter itself could also be liable because it is not only a publisher but also aggregated the content that fed the speculation.

There is an issue of legal jurisdiction: because Twitter is registered in the US, there could be protection under the free speech amendment.

Sara Mansoori, a barrister at Matrix Chambers, which has acted in a number of high-profile libel cases, said: "It is possible [Twitter] could be sued. It is a publisher, but it would be complex because of the jurisdiction argument."

Lord McAlpine's action seems to have sparked concern in the blogging community. John Ward, who writes the Slog blog, said yesterday: "I have had a lot of people desperately worried about their email and comment threads on my site. They have asked to have things taken down. One wrote to me saying she hadn't been able to eat or sleep for worry."

Mr Reid has instructed a team of experts to trawl through all the tweets and retweets that made reference to his client – some 3,000 on the day of the Newsnight broadcast, which, although not naming Lord McAlpine, helped in the jigsaw identification of the peer by referring to a senior Conservative politician.

Has Twitter fundamentally changed the law of libel, not only for individual users but also for the mainstream media? If the Newsnight programme had been broadcast 20 years ago, it is unlikely to have faced legal action – particularly as the Bureau of Investigative Journalism would have been unable to alert the public, as it did via Twitter, to the story of a "senior Conservative politician", contributing to the piecemeal identification. Social media is now, arguably, so well integrated into, and influential over, mainstream media that it is impossible to separate the two. Despite this, Lord Justice Leveson's inquiry made little attempt to investigate how social media affects the ethics of the press.

But should we be living in fear of lawyers monitoring every snide comment we make about an X Factor contestant or a hapless politician? Under section 12 of the 1952 Defamation Act, it may be problematic for Lord McAlpine to get multiple damages. According to the reputable Inforrm legal website: "It could be argued that, as the BBC had paid such a substantial sum [£185,000] to compensate Lord McAlpine for the damage to his reputation, the damages payable by those tweeting the same allegation should be very modest."

Mark Lewis, a media lawyer, said: "The court would take account of the damages that had been paid in respect of the libel, but... people need to be aware of the consequences of their actions. If you would not repeat gossip in person, don't do it on Twitter."

A leading libel lawyer said yesterday: "I don't think there's anything apocalyptic about this. It is no bad thing for people to realise that tweeting has consequences... Very often it is difficult to track people down and they may not have any money anyway, so it isn't worth going after them. What would be sensible and proportionate would be for those who have done wrong to apologise, and for Lord McAlpine to move on once they have done so."