Among his many skills and talents, Andrew Marr does not count that of forensic accounting. That was obvious – unfortunately – to anyone watching his interview with Brexit financier Arron Banks. As predicted by myself, by Carole Cadwalladr and by many others the interview failed to land a punch. Instead, it provided Banks with a platform from which to attack his accusers and muddy the waters. This was not public interest journalism, it was carnival. And it did a great deal of damage.

There are those who find such criticism of the BBC, and of its editorial decisions, distasteful. “Careful, Andrew,” they say to me, even as more often than not they agree with the substance of my case, “you don’t want to damage Auntie, we could end up with the Beeb being run by Murdoch!” Well I certainly want no such thing. But recently, and particularly on the issues surrounding Brexit, it has frankly been hard to see what difference it would make.

Marr founders on the Rocks of Arron Banks's muddy waters | John Crace Read more

The Banks broadcast is far from being an anomaly. The BBC is consistently manipulated by Brexiteers into providing them with false parity in arguments where their views add nothing, represent nobody and are demonstrably and factually wrong. Nigel Farage – the former leader of a party that polls in single figures and has no MPs – has appeared on Question Time disproportionately often. Brexit campaigners are paraded constantly across the airwaves to spout nonsense and lies about the single market, the customs union and – most unforgivably – the Irish border. Day after day BBC Radio’s flagship show – Today – descends into surrealist farce as John Humphrys seems to bark inanities at anyone presenting an evidenced argument against Brexit.

Play Video 1:37 The moment Arron Banks walks out of select committee meeting - video

On top of these errors – made by taking the name of balance in vain and not unlike the practice of inviting climate-change sceptics to debate Nobel-winning scientists – is the BBC’s institutional determination to declare the existential questions of Brexit answered.

The BBC political editor’s immediate reaction to an investigation into allegations of criminal offences by Banks – which he denies– was to write that it was “unlikely” to affect Brexit. A tad premature, to say the least. And it is because of this instinct – to get it “over and done with” – that the BBC has so consistently failed in its coverage of the campaign for a people’s vote. Farage gets more coverage for a stunt throwing fish out of a boat – accompanied by a couple of fishermen and an entire school of journalists – than the People’s Vote campaign does for any of our many regional events and action days that attract volunteers in their hundreds.

These blind spots are undermining the BBC as an institution. They have deformed its news output and led to catastrophically bad decisions – which have normalised fringe politics and given false weight to outright lies. I don’t criticise the BBC lightly or with any joy. I do so because it is getting it badly wrong – for itself and for us all. If the BBC expects to continue to enjoy public funds and public trust it needs – urgently – to examine its conscience and its output. It is not “balance” to invite contributors to mislead your audience. It isn’t “public interest journalism” to offer Banks a platform to attack his accusers and investigators. And it is not the responsibility of progressives and liberals to keep quiet in the face of such terrible judgment – either out of loyalty or from fear of what might replace the beloved BBC. It is our job to call it out.

• Andrew Adonis is a Labour peer