Countdown presenter Rachel Riley has won a High Court libel battle against a political blogger and Jeremy Corbyn supporter over an article which alleged she engaged in a “campaign of online abuse and harassment” against a teenager on Twitter.

Ms Riley sued Mike Sivier, who published an article on his website Vox Political in January 2019 with the headline: “Serial abuser Rachel Riley to receive ‘extra protection’ – on grounds that she is receiving abuse.”

Mr Sivier defended the claim by arguing it was “substantially true” that Ms Riley had “engaged upon, supported and encouraged a campaign of online abuse and harassment of a 16-year-old girl” during Twitter exchanges in December 2018 and January 2019.

But, in a ruling on Wednesday, Mrs Justice Collins Rice ruled in favour of Ms Riley and struck out Mr Sivier’s defence, concluding that it had “no prospect” of succeeding at a libel trial.

The allegation centred on tweets posted as part of the online debate on anti-Semitism in the Labour Party, some of which were exchanged between Ms Riley and a Twitter user who identified herself as a 16-year-old called Rose.

The judge said that, contrary to the allegations in Mr Sivier’s article, Ms Riley’s online discussion with Rose was a “straightforward, rational and respectful exchange of views”.

She also said there was no evidence the presenter had encouraged any of her 600,000 followers to harass Rose, nor was she responsible for any abuse the teenager received.

Setting out her conclusions, she said: “Rose challenged Ms Riley to engage with her and thanked her for her response.

“Third parties appear to have made mischief in the wake of that engagement, and Rose herself publicised a narrative that she was the victim of bullying for which Ms Riley’s ‘gang’ was to blame.

“Mr Sivier’s website article said this was fact, and he seeks now to defend that by showing Ms Riley’s speech and ‘deliberate and calculated omissions’ were responsible.

“There is no arguable source for that responsibility in law or fact; and it is a still further leap to the truth of an allegation of deliberately supporting and encouraging a campaign of abuse and harassment.

“I do not see an argument from Ms Riley’s alleged omissions realistically capable of supporting that allegation.

“For all these reasons, I have not been able to discern in Mr Sivier’s pleadings a case, arguable with a realistic prospect of success, that it is substantially true that Ms Riley engaged upon, supported and encouraged a campaign of online abuse and harassment of Rose.

“That itself precludes the possibility of arguing that any such conduct incited others to make death threats to Rose.

“If such threats were made there is no basis for saying they were incited by Ms Riley’s conduct as alleged, since there is no arguable basis for establishing the objective fact of that course of conduct.”

The judge said the backdrop to the dispute was the “debate about anti-Semitism and the Labour Party which has featured in British politics in recent years”.

She added: “At one end of a polarised spectrum was a view of the Labour leadership as insufficiently active in dealing with allegations of anti-Semitism within the party, to the point of practically condoning it.

“At the other end was a view that this critique was so unjustifiable as to amount to political smearing intended to undermine the leadership.

“The debate unsurprisingly aroused strong, sometimes personal, feelings.

“Views were expressed that some criticism of those challenging the Labour leadership to do more was itself tainted with anti-Semitism.”

She also said that Twitter was “a haunt of trolls”, adding: “The vulnerable enter at great personal peril.”

The judge said “the Twittersphere” is an arena for people to air views which “can and do offend in manner and content”.

She added: “Where personal views are exchanged in this arena, on a topic of intense and polarised political debate, and on a subject which could hardly be more sensitive in its engagement with profound issues of personal and community identity, then strong feelings will be unsubtly expressed, and offence caused and taken.”

The judge said that does “not excuse” any online harassment of the vulnerable, but nor could it mean that a well-known television personality accepts that the “price for participation in intense Twitter debates is implication in the impact of any following troll on a vulnerable person”.

She added: “Celebrity status, and a large Twitter followership, doubtless afford opportunities for role-modelling and influence.

“But ‘omission’ to take those opportunities to champion the vulnerable and condemn the intemperate, is not, even arguably, tantamount to supporting or encouraging online harassment.

“Celebrities are responsible for what they say on Twitter, but they are not responsible for the Twittersphere simply by entering it, nor for Twitterstorms which others choose for any reason – pro or anti the celebrity – to generate around them or around their online speech and contacts.”

In a post on his website, Mr Sivier said he intends to appeal against the decision and asked supporters to help fund the legal challenge.