Speaking on BBC Radio 4’s Woman’s Hour, the Labour leader was lost for words while host Emma Barnett roasted the politician for not knowing his numbers on the non-means-tested policy.
When asked for the first time, how much it would cost, Mr Corbyn replied: “It will cost – it will obviously cost a lot to do so, we accept that.”
Barnett pressed again and said: “I presume you have the figures.”
“Yes I do," he replied. "It does cost a lot to do, the point I’m trying to make is, making it universal so that we are in a position to make sure that every child gets it.”
He said it would be funded “mainly through corporate taxation” before he was repeatedly pushed for an exact figure.
Jeremy Corbyn was left floundering over his figures during the awkward interview
It’s quite troubling, this is a policy you’re launching today Mr Corbyn and you don’t know how much it’s going to cost – hardly inspires the voters
“So how much?” Barnett asked, followed by an awkward silence.
Mr Corbyn asked the presenter to give him a moment as he reached for his iPad to find the figure.
The host then berated the party leader and said: “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?”
The shocking interview continued with Mr Corbyn asking for some time once more before Barnett slammed his party and said people felt they could not “trust you with our money”.
Mr Corbyn insisted they could but Barnett said: “You don’t know the figure.”
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Snap election 2017: The pictures politicians may not want you to see Sat, May 27, 2017
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Jeremy Corbyn during a visit to Hackney Marshes Football Pitches, to highlight Labour's manifesto commitment to ensure 5% of the Premier League's television rights income is diverted to the grassroots game, during a General Election campaign
Jeremy Corbyn on the campaign trail
The politician was still unable to give a figure but maintained “all of our manifesto is fully costed and examined”.
The broadcaster could not let the issue slide and replied: “You’re holding your manifesto, you’re flicking through it, you’ve got an iPad there, you don’t know how much it’s going to cost.”
She added: “My point is, it’s quite troubling, this is a policy you’re launching today Mr Corbyn and you don’t know how much it’s going to cost – hardly inspires the voters.”
Mr Corbyn said he wanted to give an “accurate” figure but Barnett was later forced to put the cost to the Labour leader herself.
“It’s actually from Angela Rayner, your shadow education secretary, £2.7bn, and then £4.8bn, with half a billion to reverse the cuts to the Sure Start scheme. Does that sound about right?”
Jeremy Corbyn replied: “It does sound correct.”