Mr Banks brought the High Court proceedings against Ms Cadwalladr, a journalist with The Guardian and The Observer, in relation to a TED talk she gave and a tweet she posted, both in 2019.

The “single meaning” of the TED talk and the tweet was determined as a preliminary issue and held by the court to be: “On more than one occasion Mr Banks told untruths about a secret relationship he had with the Russian government in relation to acceptance of foreign funding of electoral campaigns in breach of the law on such funding.”

Under the requirements of the Defamation Act 2013, Mr Banks was required to show that the TED talk and the tweet caused serious harm to his reputation. He succeeded in respect of the TED talk but not in respect of the tweet.

The defendant relied on the public interest defence, and so the onus was on her to show that (a) the statement complained of was on a matter of public interest, or formed part of such a statement, (b) that she believed that publishing it was in the public interest and (c) that her belief was reasonable.

The parties agreed, and the court found, that the TED talk was on a matter of public interest, and the court found that the defendant believed publication was in the public interest. The major point of contention was whether the defendant’s belief that publication was in the public interest was reasonable.

The court held that the defendant had intended to convey a less serious, but still defamatory, meaning than the “single meaning” determined by the court.

Applying the Bonnick principle, the court found that the defendant did not appreciate that her words could carry the single meaning and it was not so obvious that it was unreasonable for her to have failed to appreciate it. The public interest defence fell to be assessed having regard to her intended meaning.

The reasonableness of the defendant’s belief had to be determined by reference to all the circumstances. A key factor was that the defendant had reasonable grounds to believe that her intended meaning was true.

The court therefore concluded that Ms Cadwalladr had established a public interest defence under section 4 of the 2013 Act.

It also held that there was a significant change of circumstances after statements published by the National Crime Agency (NCA) and the Electoral Commission in 2019 and 2020 respectively, and that the defendant failed to establish a public interest defence in respect of the publication of the TED talk after that latter date.

However, the court held that where a public interest defence applies to the original publication, but then falls away due to a significant change in circumstances, a fresh assessment of whether the serious harm condition in the 2013 Act is met is required from the date of that significant change.

The court was not satisfied that the continuing publication of the TED talk after that date caused, or was likely to cause, serious harm to the claimant’s reputation. Accordingly, the requirements of the 2013 Act were not met in respect of publication of the TED talk after that date, and so that part of the claim was also dismissed.

In a joint statement, Paul Webster, editor of The Observer, and Katharine Viner, editor-in-chief of Guardian News & Media, welcomed the verdict as “an important victory for free speech and public-interest reporting”.