Keir Starmer refused to rule out stealth increases in income tax or National Insurance as the Government faces a £5 billion funding black hole. He could not rule out a freeze in tax thresholds - which means working people will pay more tax because of the impact of inflation.
Conservative leader Kemi Badenoch pointed out that millions of pensioners "face being dragged into income tax for the first time ever". And she urged the Government to rule out introducing a brutal "retirement tax" by making even low-income older people pay income tax.
But speaking during Prime Minister's Questions in the House of Commons, Sir Keir refused.
He insisted that he could not comment in advance of Chancellor Rachel Reeves's budget later this year.
But last year, Ms Reeves made a clear promise to end the freeze on thresholds. She said last year: " Having considered this issue closely, I have come to the conclusion that extending the threshold freeze would hurt working people. It would take more money out of their payslips."
Mrs Badenoch asked the Prime Minister if this was still the Government's promise today and he failed to give an answer, simply attacking the Tories instead.
It comes after Labour's u-turn on benefit cuts left the Chancellor struggling to fill a hole in the public finances.
The Tory leader also pointed out that Council Tax bills are soaring.
She asked: "The Chancellor promised that she would lift the freeze on income tax and National Insurance thresholds, because in her words they hurt working people. Is that still government policy?"
Sir Keir said: "No prime Minister or Chabncellor is going to write the Budget in advance."
He added: "We are absolutely fixed on our fiscal rules. We remain committed to them.
"We remain committed to our budget, to our manifesto commitments. I realise that sticking to your fiscal rules and your manifesto commitments is a bit unfamiliar to the party opposite (the Conservatives) but it's because of the decisions the Chancellor took, that this Government has taken, that we can update the House, £120 billion of inward investment into this country since we took office, business confidence at a nine-year high, that's longer than the leader of the Opposition has been in Parliament, Deloitte this week saying the UK's now the best place to invest, creating 384,000 jobs.
"What a contrast to the mess we inherited from the party opposite."
But Ms Badenoch said: "There was no clear answer there."
And she warned: "It means that under Labour, millions of our poorest pensionsers face being dragged into income tax for the first time ever."
Mrs Badenoch: "This weak prime minister has been dragged into a series of chaotic u-turns".
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