British Labour leader Jeremy Corbyn was unable to say how much his plans to reform childcare would cost as he launched the policy, attracting strong criticism from rivals.
Mr Corbyn, who is standing on a platform of higher taxes on the wealthy, higher public spending and higher investment, stumbled over how much money it would take to broaden provision of free child care in a BBC radio interview.
"It will cost, erm, it will obviously cost a lot to do," he said during a prolonged awkward exchange. "I'll give you the figure in a moment."
Opinion polls show Labour have closed the gap on the Conservatives substantially during the General Election campaign, but Mr Corbyn's personal ratings remain low and the party has struggled to overcome long standing doubts about its fiscal competence.
Footage of the interview then showed Mr Corbyn thumbing the pages of his own manifesto and logging in to an iPad as he was repeatedly asked to provide a cost.
Corbyn unable to recall cost of childcare reform in interview https://t.co/CsIGDAYRNJ pic.twitter.com/ZUKbkorlLo
— RTÉ News (@rtenews) May 30, 2017
He said: "I think what is important for the voters to understand is that if we don't invest in our children, and we don't invest in them for the future then they do less well in primary school, less well in secondary school, and less well in the future."
He said all of Labour's election polices were fully costed.
The rival Conservative Party said it showed he was a risky choice as leader.
Responding to criticism at a later campaign speech, Mr Corbyn said: "I didn't have the exact figure in front of me so I wasn't able to answer that question, for which obviously, I apologise."
Prime Minister Theresa May has also faced criticism over funding in her party's manifesto, after it was published without a detailed financial breakdown.
Polls open in the election on 8 June.
May and Corbyn set out opposing Brexit deal stances
In the main televised event of the campaign last night, Mrs May has said she would walk away from divorce talks with the European Union without a deal if she had to, but Mr Corbyn said he would make sure an agreement was reached if he won power.
The General Election will decide whether Mrs May or Mr Corbyn will get to sit down with Brussels and hammer out an exit deal that will define the country's trade and diplomatic ties with the EU.
Their differing stances could set the tone for what both Britain and the EU expect will be two years of difficult talks on everything from how much cash should be paid upon exit, to border arrangements for migrants, goods and services.
Mrs May is expected to win comfortably, but her party's lead in opinion polls has narrowed sharply in the last week, calling into question her decision to call the unscheduled election seeking a strong endorsement of her Brexit strategy.
The two party leaders' differences on how to handle talks with Brussels came to the fore during the televised event, which saw them separately field questions from the public and then interviewed.
"We will be there to negotiate the right deal but what I have said is that no deal is better than a bad deal. We have to be prepared to walk out," Mrs May said to applause during the Sky News interview.
She has previously set out her willingness to walk away from talks, warning that Brussels could seek to punish Britain to deter other EU states from leaving by taking a hardline stance.
But, Mr Corbyn set out a different approach.
When asked whether he would contemplate a scenario where Britain failed to strike an arrangement with the rest of the bloc, Mr Corbyn said: "There's going to be a deal. We will make sure there is a deal."
With polls showing Mrs May is more trusted to deliver Brexit than Mr Corbyn, she will look to keep the election debate tightly focused on the upcoming talks, seeking to recover from a backlash to her plans to make elderly voters pay more towards the cost of their old age care.
