Theresa May says a Conservative government will not raise VAT if she wins the general election.
But the PM declined to back a 2015 Tory pledge that income tax, National Insurance and VAT would not be increased.
Mrs May said she would not be making “specific proposals” on taxes unless she is “absolutely sure” they can be delivered.
She added that her intention was to cut taxes for working families.
Asked whether she would be repeating ex-PM David Cameron’s “five-year tax lock”, covering income tax, National Insurance and VAT until 2020, Mrs May told the BBC’s Andrew Marr Show: “We have absolutely no plans to increase the level of tax.
“But I’m also very clear that I do not want to make specific proposals on taxes unless I am absolutely sure that I can deliver on those.”
Speaking later on the Peston on Sunday show, she added: “We have no plans to raise the level of tax.
“In relation to specific taxes, we won’t be increasing VAT.”
Labour said the Tories had previously increased VAT having promised not to do so.
Shadow chancellor John McDonnell told the Peston show there would be no increases in income tax for “low and middle earners” under Labour, with details to be set out in his party’s manifesto.
“The only increases will be on that higher percentage,” he said, adding that Labour’s manifesto would be fully costed.
On the Marr show, Mrs May suggested the “triple lock” protecting the state pension could be changed, saying state pensions would continue to rise, with the exact method of calculations to be revealed in the Tories’ manifesto.
The triple lock ensures the state pension increases in line with wages, inflation or by 2.5% – whichever is highest.
And she rejected claims she was “in a different galaxy” to the rest of the EU on Brexit negotiations.
After the 27 EU leaders agreed their Brexit negotiations, the PM said there was “good will” on both sides about settling the issue of EU nationals living in the UK, and Britons elsewhere in Europe.
She said recent comments by EU figures showed that talks could be hard, and stressed her determination to reach agreement on a trade deal at the same time as settling the terms of the UK’s exit.
Lib Dem leader Tim Farron told the Marr show the UK “desperately needs a strong opposition”, saying that Lib Dem success was the only way to prevent the “calamity of hard Brexit”.
A Conservative majority was “not in question” and Mrs May was heading for a “colossal coronation”, he predicted.
He said the decision to call the general election had been motivated by a “cold, calculated desire to do what’s best for the Conservative Party”.
Plaid Cymru leader Leanne Wood told the Sunday Politics show any Brexit plan that threatened Welsh jobs would be “extreme” and “unacceptable”.
Former SNP leader Alex Salmond said his party would provided “real opposition” to the Tories on public spending cuts.