Labour has struggled to spell out how it will raise the £63 billion to go on its promised spending spree if it gets voted into government.
Shadow business secretary Rebecca Long-Bailey was repeatedly challenged over how her party will raise the cash while sticking to its promise not to increase borrowing.
She said independent research showed that reverses to a series of tax cuts would raise £70 billion by 2020, but the BBC's Andrew Marr said analysis of official figures shows a £30 billion black hole in the party's funding plans.
Mr Marr said: "The cost is all based on either red book numbers – official numbers – or Labour's own costing and it comes to something like £60 billion of extra spending.
"Now you have also said that your fiscal credibility rule means you will not borrow to do any of this, so my question is very straightforward – where does the money come from?"
Ms Long-Bailey said: "Well, we certainly wouldn't have made the decisions that this Government has, for example slashing taxes for the most wealthy in society – inheritance tax, capital gains tax, the bank levy, corporation tax."
But challenged by Mr Marr about how much Labour would raise by reversing corporation tax, the shadow business secretary was unable to come up with a figure.
She said: "As a total package, we asked the House of Commons to do some research in terms of the money that we would gain back if we reversed all of those tax breaks.
"As whole it is £70 billion in total by 2020."
Repeatedly pressed on how the £70 billion figure broke down, the Labour frontbencher struggled to come up with an answer.
Mr Marr said: "You are giving me a meaningless overall figure, if you don't mind me saying so, I just want to know in detailed terms."
Official figures analysed by the BBC show that Labour would raise £30 billion from all the tax cut reverses, leaving a gaping hole in the Labour numbers, Mr Marr said.
"Corporation tax, according to the budget, gets you £17 billion, raising the inheritance tax will get you just £2.8 billion, capital gains tax cuts cost just under £3 billion, again according to the budget, and receipts from the bank levy are forecast to be only £4 billion lower by 2020 compared to the OBR estimates," he said.
"Again these are all official figures, tot it up together and it comes to £30 billion not £60 billion, so you are still £30 billion short. And my question is, if you are talking about credibility if fiscal credibility really matters to you, where does that money come from?"
But Long-Bailey stuck to her answer, telling the show: "No, it's inheritance tax, capital gains tax, corporation tax and the cuts to the bank levy which total £70 billion by 2020."