You realise the depth of the mess the Labour Party is in when shadow chancellor John McDonnell mounts a vigorous defence of Jeremy Corbyn’s performance at today’s Prime Minister’s Questions. I suppose you would expect his closest political ally, the real power behind the throne, to support his ailing leader, but this time Corbyn gave easily the worst performance of his wretched career as Labour leader, no easy feat given the competition. McDonnell’s intervention was closer to parody than common sense.

Picture the scene. A packed House of Commons, the chancellor having earlier made a dramatic, screeching U-turn on whacking up National Insurance contributions for the self-employed just a week after his budget, questions to a wobbly prime minister who is not good at PMQs and Jeremy Corbyn, yet again, faces a massive open goal. How could he possibly fail? But fail he did. He didn’t just do a Ronny Rosenthal (look at you tube, kids), he smashed the ball against the crossbar, picked up the ball, went down the other end and smashed the ball past his own goalkeeper.

Corbyn stood there with six questions to ask, but he didn’t really ask any questions: he started to rant and rave. It is hard to underestimate the seriousness of Philip Hammond’s U-turn and the disunity on the Tory benches. Extra taxes on entrepreneurs goes against everything the Tories are supposed to stand for. Hammond tried to impose extra taxes and a week later he caved in. The leader of the opposition went around the houses, somehow managing to change the subject and find himself talking about school books. Theresa May couldn’t believe her luck and hurled a few insults, along with the obligatory references to “ordinary working people” and a “country that works for everyone”. It’s so easy.

The most amazing thing is that Corbyn didn’t even call for Hammond to resign. I am not saying that Hammond would have then resigned, but politics is about building pressure. Focused, forensic and persistent questioning would surely have damaged both May and Hammond, but that’s not what you get from Corbyn. If anyone was in any doubt about just how useless he is, they needed to see PMQs today.

McDonnell saw Corbyn today – he was sitting next to him, for goodness sake – and it sounded absurd when he defended him. There are two reasons for this: either McDonnell really did think Corbyn performed well which suggests he has a common sense blind spot or he is lying to protect his leader. McDonnell is many things and not many of them are very nice, but he is not stupid. If he took a lie detector test I’d wager a considerable sum of cash that he’d fail it if asked whether Corbyn was any good.

In fact, McDonnell became increasingly tetchy with the Radio Five Live presenter Anna Foster as the interview went on. He started with his soft, gentle voice but once Foster started to ask the awkward questions, out came the nasty piece of work McDonnell really is. The interview ended with McDonnell coming out with his usual drivel about “BBC bias”, as if a journalist should not be allowed to mention Corbyn’s PMQs car crash. Yes, there are sometimes issues with the BBC, such as their obsessive need to have Nigel Farage appear on Question Time every other week or to appear on TV and radio at the drop of a hat, but when it comes to the current Labour leadership, the problems are all of their own making.

I urge you to judge for yourself. Watch today’s PMQs yourself. May was poor, as usual, but she doesn’t need to be better than poor when she is up against Corbyn. If you think Corbyn did well, you are either John McDonnell or a Tory supporter. Theresa May surely can’t believe her good luck.