Theresa May has refused to rule out abandoning a 2015 election manifesto pledge not to raise taxes.
Speaking on the campaign trail in Dudley, in the West Midlands, the prime minister would not be drawn on the issue when asked if the 2015 pledge not to put up income tax, VAT or national insurance would be included this time.
However, May insisted that the Conservatives would remain the party of "lower taxes".
She said: "At this election, people are going to have a very clear choice.
"They will have a choice between a Conservative party, which always has been, is and will continue to be a party that believes in lower taxes... [or] a Labour party whose natural instinct is always to raise taxes."
She also declined to guarantee existing spending on state pensions.
May's comments came after Philip Hammond seemed to hint that the pledge could be ditched.
The chancellor, who recently had to U-turn on a planned national insurance raise, said that the 2015 manifesto had constrained "the ability to manage the economy flexibly".
Speaking in Washington DC on Friday, he said: "I'm a Conservative. I didn't come into politics because I believe in higher taxes. I'm not in the business of having an ideological desire to raise taxes.
"But we need to manage the economy sensibly and sustainably. We need to get the fiscal accounts back into shape.
"It was self-evidently clear that the commitments that were made in the 2015 manifesto did and do today constrain the ability to manage the economy flexibly."
Shadow chancellor John McDonnell has accused the Tories of planning a "tax bombshell" if they are re-elected.