Media playback is unsupported on your device Media caption Watch: Jeremy Corbyn on Woman's Hour

Jeremy Corbyn was unable to put a cost on Labour's plan for free childcare for 1.3m youngsters during an interview with BBC Radio 4's Woman's Hour.

"It will cost... it will obviously cost a lot to do so, we accept that," he said, before agreeing with host Emma Barnett that the figure was £5.3bn.

The stumble came as the Labour leader and Theresa May resumed election campaigning following a live TV debate.

Mrs May has argued she is the only one who can offer "strong leadership".

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It was during his interview with Woman's Hour that Mr Corbyn had to be told by interviewer Emma Barnett the cost of his party's manifesto proposal to extend 30 hours a week of free childcare to all two-year-olds - a policy which is part of Labour's plans for a National Education Service.

BBC political editor Laura Kuenssberg said that Mr Corbyn's performances had been getting better during the election campaign, so it was "astonishing he was not across the figures for the childcare policy" his party was campaigning on today.

The childcare part of the interview began with Mr Corbyn being asked whether he had the figure for its cost and replying: "Yes, I do."

Pressed to give the number, he said: "I'll give you the figure in a moment."

"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?" presenter Emma Barnett said.

"Can I give you the exact figure in a moment, please?" the Labour leader said.

Asked whether this indicated that voters should not trust Labour with their money, he answered: "Not at all."

He argued that investing in children in early years meant they did better in school and added: "I want to give you an accurate figure."

Eventually, Ms Barnett quoted shadow education secretary Angela Rayner, who was on BBC Radio 4's Today programme earlier: "£2.7bn, then £4.8bn... with half a billion to reverse cuts to the Sure Start scheme. Does that sound about right?"

"It does sound correct," Mr Corbyn said.

Image copyright PA Image caption Jeremy Corbyn is promoting Labour's childcare policy on Tuesday

Analysis - BBC assistant political editor Norman Smith

This was on a par, or possibly even worse, than Diane Abbott getting into a pickle on the cost of Labour's policing pledges.

That's not just because Jeremy Corbyn is the party leader, but also because childcare is an absolutely key pledge, he's campaigning on it today - and he was clearly struggling badly for the numbers.

When you get the leader struggling to say how much a core Labour policy is going to cost it sparks questions about how clear, how thorough, how credible its spending plans are.

During a later event in Watford, Mr Corbyn apologised for failing to recall the cost of his party's childcare policy.

"I didn't have the exact figure in front of me, so I was unable to answer that question, for which obviously I apologise," he said, adding that it was "unacceptable" that Ms Barnett had reportedly been subject to abuse on social media for her grilling of the Labour leader.

Media playback is unsupported on your device Media caption Jeremy Corbyn apologises for childcare costs stumble

The Labour leader has put the creation of a "national education service" at the heart of his party's election campaign.

The extension of childcare would benefit more than 1.3 million children - at the moment only 40% of two-year-olds qualify, according to Labour.

Labour's shadow international trade secretary Barry Gardiner defended Mr Corbyn's stumble over the cost of his party's childcare policy, telling BBC2's Daily Politics: "It wasn't that the figure wasn't there. It wasn't the figure hadn't been costed. Look this is a rapid fire general election campaign in which people, politicians are under spotlight."

Media playback is unsupported on your device Media caption Barry Gardiner challenges Conservative Michael Gove on costing of some of his party's policies

During the Woman's Hour interview, Emma Barnett also asked Mr Corbyn about Labour's manifesto pledge to renew Trident, despite CND's website saying that he was still vice president.

The Labour leader, who voted against the renewal of Trident, said he was a member of CND, but added: "I don't think I'm vice president".

For the Conservatives, International Development Secretary Priti Patel said Mr Corbyn's "chaotic radio interview with Emma Barnett" showed the British people "why he is simply not up to the job of being the prime minister".

Media playback is unsupported on your device Media caption Jeremy Corbyn shared some childhood photos with the One Show

Mr Corbyn was interviewed on the One Show on Tuesday evening, where he said the prospect of becoming prime minister did not fill him with fear but "hope of what we can do and what we can change in this country".

He also made a joking reference to strife within the party, saying 10-year-olds were easier to control than Labour MPs.

The interviews followed a high-profile grilling in a Channel 4/Sky News TV special on Monday when Mr Corbyn, and Conservative leader Theresa May, faced questions on range of subjects, including Brexit and foreign policy, from a live studio audience in the special general election programme on Monday night.

Media playback is unsupported on your device Media caption The two leaders faced audience questions on the NHS and the economy

Media playback is unsupported on your device Media caption Theresa May and Jeremy Corbyn were also questioned by Jeremy Paxman

Mr Corbyn was quizzed about his views on drone strikes, tax-raising plans and past campaigning in Northern Ireland.

Mrs May was questioned about social care reforms and was repeatedly asked if she had changed her mind on Brexit.

But the leaders did not appear together, as Mrs May declined to take part in a head-to-head encounter.

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