Shadow business secretary clashes with Andrew Marr over Labour's spending plans
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."
It comes as a row engulfed the Labour Party after Jeremy Corbyn angered his MPs by telling the Press Association he is "absolutely fine" for a second Scottish independence referendum to go ahead.
Labour MP Hilary Benn, who chairs the Brexit select committee, told ITV's Peston on Sunday that it is Labour policy to oppose another independence referendum.
Asked if he agreed with Mr Corbyn that a second vote should take place, Mr Benn said: "No, our position, the position of the Labour Party in Scotland, is that we are not in favour of a second referendum, and that the most important point to make is that all the indications are that the people of Scotland are not in favour of a second referendum. That's what the polling shows.
"And we have enough uncertainty with Brexit."
Challenged over how long Mr Corbyn can stay leader while flouting official party policy, Mr Benn said: "Look, Jeremy has set out his view, but the policy of the Labour Party is that we do not support (a second referendum).
He said Kezia Dugdale, leader of the Scottish Labour Party, will fight against another referendum.
Sir Keir Starmer, the shadow Brexit secretary, also distanced himself from Mr Corbyn's original comments.
He told Sky News' Sophy Ridge On Sunday: "I support the approach that Kezia Dugdale, our leader in Scotland has been arguing for years.
He said Mr Corbyn has put out a statement clarifying his position, adding: "And Kezia has been clear - no second referendum but focus on what really matters for Scotland at the moment."
A shadow Treasury spokesman said: " The £70 billion is a conservative figure that comes from Labour research in consultation with the House of Commons Library. It includes the effects between now and 2022 of the changes to taxes made by the Tories since they came to power.
"This represents a massive giveaway to the rich and corporations, especially unacceptable at a time our NHS is in crisis and families are being hit with cuts in tax credits.
"The Tories are hitting low and middle earners while giving away over £60 billion in corporation tax breaks alone in the coming years.
"When you add in Capital Gains Tax, inheritance tax, their failure on the bank levy and on tackling tax avoidance the scale of their unfair choices becomes clear. Labour would make different choices.
"We will set out our detailed plans at the election but one thing is clear: unlike the Tories we will put ordinary people first, not the few at the top."