The claim that Jeremy Corbyn is unfit to be prime minister has apparently come as a shock in some quarters. The Labour leader has found himself the weekend target of anonymous civil servants who claim he is losing his memory and is too frail to be prime minister.

Such attacks are counter-productive. Not only do they encourage his supporters to cry “establishment plot” and throw a protective cordon around their leader, they detract from the real issue which is that over recent months he has been decisively proving he is not up to the job. In his vacillation over Brexit, weakness on anti-Semitism and his inability to grasp the agenda from a hapless government, Mr Corbyn has shown an utter lack of leadership. It is not because he is unwell; it is because he is no good.

Even one-time allies are starting to despair. Clive Lewis, Treasury spokesman, said Mr Corbyn needed to “get a grip” over Brexit. Angela Rayner, shadow education secretary, said the top team was “totally exasperated” by the most recent anti-Semitism row.

In the latest YouGov poll of who would make the best prime minister, Mr Corbyn is 12 points behind Theresa May. The most recent poll of voting intentions shows Labour losing a third of its support in the last two months. The party risks blowing its best chance of power for a decade.

What is so damaging about the health rumours is that they play to an existing public perception of a man simply not of prime ministerial timbre. Whispers about Mr Corbyn’s health have swirled around Whitehall for months. Officials gossip, without evidence, that his eye condition was a mini-stroke. Others note how unengaged he is in meetings. One Labour figure recounted a meeting on a major problem in which Mr Corbyn suddenly wandered off the subject into a disquisition on tenant farmers. MPs contrast this with the focus and grip of John McDonnell, the shadow chancellor.

But Mr Corbyn has always been like this. Those who complain he is a prisoner of a cabal in his office are missing the point. He was from the start a titular figurehead to the project. He is interested in broad themes — class struggle, foreign policy. His view is always aerial and detail left to others.

There is no dispute about the undue power and bullying style of those who surround the leader — Seumas Milne, his press chief, Karie Murphy, chief of staff, Len McCluskey, head of the Unite union, and Andrew Murray, Unite’s chief of staff. But while MPs see him as a prisoner, he sees it as protective custody against a hostile parliamentary party.

But his aides are highly factional. Their priorities are securing the party and the succession (shadow business secretary Rebecca Long-Bailey is the current favourite) as much as they are to win power. Allies are backed, however heinous their offence. All issues — Brexit included — are secondary to building a socialist Britain.

After the European elections the Labour leader appeared ready to shift towards a Remain strategy, at least pledging a second referendum. Shadow cabinet members expected a decision last week. Instead they got more delay as his aides fought back against a shift.

Labour’s initial policy of respecting the referendum while seeking a vague but gentler EU exit was logical. A bad Tory Brexit offered the best chance of winning power. Mr Corbyn also has up to 30 Labour MPs who feel bound not to oppose Brexit. Now, the middle ground is eroding and Labour is losing support to both the Liberal Democrats and the Brexit party.

Asked whether the resistance to a second referendum was principled or tactical, one very close Corbyn adviser replied: “It’s 100 per cent tactical.” Mr Corbyn’s circle believe they need to hold their nerve and wait for Brexit to happen. Then Labour can return to its preferred ground of economic transformation while the Tories self-destruct. Unfortunately the electoral coalition built in 2017, when Remainers lent Labour their votes, is breaking down. Those who turned to him two years ago want political and moral leadership. On the biggest issue, they are not seeing it. (This may also affect the succession, if members blame the Corbynites for not aggressively fighting Brexit). And even beyond Brexit, it is hard to recall a blow he has landed on a broken government.

Of course political choices do not exist in a vacuum. If Labour is the only real alternative to a Tory government which has delivered a disastrous Brexit, then it may win power by default.

Mr Corbyn’s enemies have failed to remove him, while his allies continue to stand by him, preferring to plough on with a man who is more cipher than leader, while hoping the Tories implode. Yet increasingly Mr Corbyn looks like a man who has been rumbled by the voters. His personal health may be unclear, but as for Labour’s political health — like a fish, it is rotting from the head.

robert.shrimsley@ft.com