John McDonnell faced claims he was forced to Google the deficit during a live radio interview today.

The shadow chancellor suffered a 'Diane Abbott moment' while being grilled by Nick Robinson on the Today programme this morning.

Mr McDonnell appeared stumped when Robinson challenged him to tell listeners what the deficit in the public finances was.

The presenter asked if he was looking it up when rustling could be heard over the radio while social media users pointed out the figure Mr McDonnell came up with matched that given on Wikipedia.

As he regained his composure, Mr McDonnell declared the gap between the government's revenues and spending was around £70billion.

But he was told that the true figure is actually £52billion.

John McDonnell has been conducting a round of broadcast interviews ahead of Labour's manifesto launch later today

The embarrassing gaffe comes after shadow home secretary Diane Abbott floundered as she tried to explain how much a pledge to boost police numbers would cost.

Mr McDonnell was conducting a round of broadcast interviews ahead of Labour's manifesto launch.

The party has proposed the most left-wing election platform for decades - including swingeing tax rises on business and the wealthy and a wave of nationalisations.

But Mr McDonnell was admitted today he did not know how much it would cost to take water companies - worth many tens of billions of pounds - into public ownership.

He suggested parliament could 'set a price' and there could be a compulsory conversion of shares into bonds.

'We will seek to negotiate and we'll look at the options as to how we bring it back into public ownership for public benefit,' he said.

'One is yes an outright purchase and we wouldn't want to set the price of that now because that would affect the negotiations.'

The shadow chancellor suffered a 'Diane Abbott moment' while being grilled by Nick Robinson on the Today programme this morning

Challenged by Nick Robinson on BBC Radio 4's Today programme that the cost would be in the 'tens of billions', Mr McDonnell said: 'A determination would be made by Parliament itself it has the right to do that.

'But also we could offer bonds for shares and exchange scheme there are different mechanism that can be used….'

He added: 'We wouldn't want to discuss any form of price at this point in time.

'As I say Parliament as we have seen as a result of past legal actions and legislation, parliament has the ability to set the price at which any public ownership method like this will be designated.'

Mr McDonnell admitted the costing will not be in the manifesto.'We will consult on the mechanism for going forward on this,' he said.

He also did not deny estimates from the respected Institute for Fiscal Studies that the spending commitments amounted to £75billion a year, insisting Labour would give details later.

Mr Corbyn - pictured campaigning in Yorkshire yesterday - will vow to target the 'tight-fisted' rich as he launches the manifesto later

Tory candidate Bob Neill expressed disbelief at Mr McDonnell's failure to give figures today

As Mr McDonnell was asked on the Today programme to give a figure for the deficit, it sounded like someone handed him a sheet of paper.

But when presenter Robinson suggested that had happened Mr McDonnell - who was speaking down the line from Bradford - denied it.

The left-winger then said that the 'deficit at the moment is £68-70billion'.

But Robinson told him he was being 'a bit kind' and the figure was actually £52billion.

Aides to Mr McDonnell insisted he had been using the 'full final' figure from 2015-16, even though a 2016-17 number has been published.

JOHN MCDONNELL'S DEFICIT MUDDLE Nick Robinson: What is Britain's deficit at the moment? John McDonnell: If I can say to you, if I can say to you, that what's happening as well in terms of day to day expenditure we need the investment, we need the investment, we need the investment…. (Noisy rustling in the background) NR: Is somebody passing you a piece of paper? What is our deficit at the moment. JM: Not at all. NR: It just sounds like someone is passing you a piece of paper handed across there. What is our deficit at the moment? JM: Our deficit is around £68 – 70 billion. NR: I think you are being a bit kind actually, I think it is £52billion is the latest figure. JM: Oh well the government estimate is that it was £68-70billion. Advertisement

Speaking on ITV's Good Morning Britain earlier, Mr McDonnell was grilled on why Labour wanted to spend billions renewing the Trident nuclear deterrent when Jeremy Corbyn would never use it.

Pressed by host Piers Morgan on whether he personally would deploy the weapons in the last resort, Mr McDonnell replied: 'I don't believe that their use is either right or, to be frank, in any way could be seen to be effective because innocent people will die.

'What we will be doing is making sure that we negotiate the reduction in nuclear weapons across the world, deescalate any forms of tension so it never arrives at that, and I don't think we'll ever arrive at that situation, but let's be absolutely clear – the Labour party policy is to renew Trident.'

On the deficit figure, an aide to Mr McDonnell said: 'John was referring to the last year for which we have full final confirmed outturn data which is 2015-16.

'That's based on the latest available OBR document which included only provisional estimates for 2016/17.'

The manifesto being published this morning is expected to include £3,000 income tax hike for higher earners to fund a Labour spending spree.

Launching the document - a version of which was embarrassingly leaked last week - Mr Corbyn vow to target the 'tight-fisted' rich.

He told the event at Bradford University they had pocketed 'tax cuts and bumper salaries while millions have struggled and been held back'.

His plans include a 50p top rate of tax for those earning more than £123,000 and slashing the threshold for 45p tax from £150,000 to just £80,000.

There would also be a percentage charge on firms that pay senior staff wages of more than £330,000.

Energy, rail and water systems would all be renationalised, under the plans.