Theresa May has admitted she does not know how many pensioners will lose their winter fuel allowance if the Conservatives win the election.
The prime minister also failed to explain how her government would find the extra £8bn a year for the NHS promised in the Tory manifesto.
May was grilled by the BBC's Andrew Neil at the end of a day in which she had already performed an embarrassing U-turn on her party's plans to shake up the social care system.
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The Conservatives have said that they will means test the winter fuel allowance - which is worth up to £300 a year for all pensioners - if they are re-elected on 8 June.
Experts have estimated that as many as 10 million elderly people could be hit by the proposal.
But asked for her own estimate of how many will lose out, May would only say that the threshold at which the changes would kick in will be decided in a post-election "consultation".
"We will means test winter fuel allowance but once again we will consult, we will ask people, charities, organisations at what level that should be set," said.
Andrew Neil said: "So you don’t know. Pensioners watching tonight, they won’t know."
The prime minister replied: "I’ve also answered that what we’re doing, Andrew, is going to be talking to people about this. Asking their views on where this should be set, not just setting it here in the studio in the Andrew Neil Interview, but actually talking to charities and organisations and consulting on it."
The Tory manifesto has promised to pump an extra £8bn a year into the NHS, but Mrs May was unable to say if that would be new money or where it will come from.
She said: "It is an extra 8 billion that is going to go into the National Health Service, but I’ve identified some areas where we will be changing the way money is used, but crucially what you need to do be able to ensure that you can fund the NHS is the strong and growing economy. As I say, our economic credibility is not on the line, it’s the Labour Party that’s in the dock on that."
Elsewhere in the bruising encounter, the prime minister repeatedly refused to say what she meant when she said the UK would face "dire consequences" if it failed to get a good Brexit deal.
She was also forced to deny claims by George Osborne that her promise to bring net migration down to the tens of thousands is opposed by every senior Cabinet minister.
"People do support the immigration target and what they’re supporting is the view of the British people," said Mrs May. "That’s what we’re supporting. Because the British people want to see us controlling migration."
In her social care climbdown earlier today, May said that the Conservatives would look to cap care costs - something which was ruled out in the manifesto and by a succession of ministers since its launch last Thursday.
She repeated her claim that she had been forced to clarify her party's position because Jeremy Corbyn had been making "fake claims" about the policy.
Neil said: "So Jeremy Corbyn is now rewriting your manifesto?"
May replied: "We have not rewritten the manifesto. The principles on which we have based our social care policy remain absolutely the same. We need to ensure that we have long term sustainability in social care. We need to be able to ensure we can fund social care for the future. We’re doing the honest thing about putting a proposal to the British people, and they will make their choice on that."