Last week the Media Reform Coalition published research into the news media's treatment of Jeremy Corbyn. It was the third such report to have been produced in recent months. The first, published by the Media Reform Coalition late last year, examined newspaper reports during Corbyn's first week as Labour leader, which it found to be overwhelmingly negative.

That was followed by a much more extensive piece of research carried out by academics at the LSE's Department of Media and Communications, published last month. That research affirmed those earlier findings and concluded that "most newspapers [had been] systematically vilifying the leader of the biggest opposition party, assassinating his character, ridiculing his personality and delegitimising his ideas and politics."

The latest report, produced by the Media Reform Coalition jointly with the Department of Film, Media and Cultural Studies at Birkbeck, University of London, focuses on the coverage of the attempted "coup" against Corbyn which followed the Brexit vote. It reported similar findings on the press, but is particularly notable for being the first systematic examination of television coverage of Corbyn and his supporters.

Its most striking findings relate to the BBC. The researchers' quantitative analysis of BBC News at Six shows that critics of Corbyn were given twice as much airtime as his supporters, and that the issues mobilised by his critics were given much greater prominence. The researchers also noted the pejorative language BBC reporters used to describe Jeremy Corbyn, his team and his supporters.

Allegations of BBC bias are so ubiquitous that it is easy to become cynical about such claims. But given the centrality of the news media, and the BBC in particular, to British political life, it is important that such debates do not collapse into nihilism.

It is true that any political party or movement will seek advantageous media coverage. Similarly, it will likely detect "bias" when the news media offers a platform to alternative or opposing views, or reports on events or statements which are likely to be to its disadvantage.

It is also important to recognise that even if all parties perceive the BBC to be biased against them, or at least claim as much, it does not follow that all such claims are equally valid and accurate. Defenders of the BBC often note that the Corporation gets attacked from "all sides". But they then tend to treat this as though it were evidence of balanced reporting. This argument is misconceived. It is rather like claiming that if two people who disagree with each other both say you are wrong, then you must be right.

One of the problems here is that more powerful political actors have far more resources at their disposal to mobilise claims of "bias". Perhaps more fundamentally, they have more ability to shape the political climate against which balance and impartiality are assessed. This is why it is important for any serious discussion of "bias" to be informed as far as possible by evidence. It is why academic research is so crucial. This latest report should be viewed in the context of decades of scholarly research, which has consistently found that BBC reporting favours the interests of powerful groups in society.

Regrettably, the latest evidence of this has been treated in a remarkably dismissive fashion by the BBC. A spokesperson describing the Media Reform Coalition as a "vested interest group". This is not acceptable. The Media Reform Coalition, with which I have been rather loosely associated (I once co-edited a partner website), campaigns for a more pluralistic media and for more ethical journalism. Referring to it in this way is rather like dismissing Oxfam research on poverty, or Greenpeace research on environmental abuses, on the basis that those organisations campaign on these issues.

Evidence needs to be taken seriously. The findings of this latest report are a real cause for concern. Had the research solely examined BBC News then the Corporation might have been able to respond that its negative treatment of Corbyn simply reflected what was going on - that accurately reporting on the vote of no confidence by Labour MPs and the wave of resignations naturally meant its output would appear "biased" against Corbyn. But crucially, the research compared the BBC's reporting with that of ITV's evening news, where the airtime allocated to critics and supporters of Corbyn was broadly balanced, and the prominence given to particular issues was less dramatically weighed against Corbyn.

Further research may reveal how far these patterns of reporting hold across other programmes, and other reporting periods. But as things stand, the BBC has questions to answer.

Tom Mills is a London based sociologist. His book, The BBC: Myth of a Public Service, will be published by Verso in November.