In an excruciating interview on Radio 4's Woman's Hour the far Left Labour leader desperately floundered as he tried to explain his confused policy on free childcare for 1.3 million two- to four-year-olds.
The embarrassing event came on a day where his campaign was specifically focussing on the childcare pledge and he had just come from a childcare centre to highlight the policy.
It had echoes of shadow Home Secretary Diane Abbott's disastrous interview earlier in the campaign when she could not say how much extra police officers would cost and at one point suggested they would be paid £30 each a year.
Can we come back to that in a moment?
The day of campaigning then got worse for Mr Corbyn who admitted he did not know if he was still vice president of the Campaign for Nuclear Disarmament (CND), having previously promised to resign.
Then Mr Corbyn's hard Left fanatical supporters took to social media to abuse Emma Barnett, the BBC journalist who had exposed his ineptitude.
The attacks included a number of anti-semitic remarks reviving concerns that many of Mr Corbyn's supporters have a hatred of Jews.
Jeremy Corbyn was interviewed on Radio 4s Woman's Hour
On Woman's Hour, Mr Corbyn paused several times when repeatedly asked to give a cost for the pledge, before asking: "Can we come back to that in a moment?"
Ms Barnett asked: "Let me understand then, how much will it cost to provide un- means tested childcare for 1.3 million children?"
Mr Corbyn, who was visibly flicking through his notes, replied: "It will cost … it will obviously cost a lot to do so, we accept that."
Ms Barnett said: "I presume you have the figures?"
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He replied: "Yes, I do. It does cost a lot to do it, the point I'm trying to make is that we're making it universal so that we are in a position to make sure that every child gets it and those that can, at the moment get free places will continue to get them, those that have to pay won't and we'll collect the money through taxation, mainly through corporate taxation."
Ms Barnett asked again, at which point Mr Corbyn said: "I'll give you the figure in a moment."
Mr Corbyn was visibly flicking through his notes
She went on: "You don't know it. You're logging into your ipad here. You've announced a major policy and you don't know how much it will cost?"
Mr Corbyn replied: "Can I give you the exact figure in a moment? All of our manifesto is fully costed and examined."
She continued: "You're holding your manifesto, you're flicking through it, you've got an ipad there, you've had a phone call while we're in here and you don't know how much it's going to cost."
Mr Corbyn said: "Can we come back to that in a moment?"
Ms Barnett said it was "quite troubling" Mr Corbyn did not know the cost, adding "It hardly inspires the voters."
Ms Barnett later informed Mr Corbyn of the figure during the interview.
Corbyn looks crushed following by-election defeat Fri, February 24, 2017
Labour leader Jeremy Corbyn leaves his home following a humiliating blow by the Conservatives after snatching Copeland in a by-election victory. The struggling politician was talking on the day his party lost a by-election to the government, the first time this has happened in over 30 years.
Adam Gray / SWNS.com 1 of 10
Jeremy Corbyn is seen at Savoy Place in London where the Labour leader was giving a speech on Brexit
She said: "It's a staggering cost; would you like to know how much your policy is going to cost, Mr Corbyn?"
The Labour leader replied: "What is your estimate?"
Ms Barnett said: "It's actually Angela Rayner, your shadow education secretary – £2.7 billion and then £4.8 billion plus that with half a billion to reverse the cuts to the Sure Start scheme. Does that sound about right?"
Mr Corbyn responded: "It does sound correct."
Mr Corbyn paused several times when repeatedly asked to give a cost
But later in an interview for Mumsnet it turned out the cost was £5.3 billion.
At a rally in Watford later in the day, Mr Corbyn apologised for his gaffe saying he "did not have the figure to hand".
He also condemned the anti-Semitic attacks on Ms Barnett by some of his supporters.
The Tories said that that the farcical events showed that Mr Corbyn was "unfit" to be Prime Minister.
International Development Secretary Priti Patel said: "In this shambolic interview he's made Diane Abbott's grasp of detail look impressive.
"This car crash interview shows Jeremy Corbyn isn't up to the job of leading our country through the challenges ahead – he is simply too big a risk to take."