Jeremy Corbyn has claimed the Conservatives' tax plans are in "chaos" after Theresa May failed to back a senior minister who said higher earners will not face an income tax hike under a new Tory government.
Defence Secretary Sir Michael Fallon said voting Conservative in the General Election on Thursday was "the only way" people could be sure income tax would not go up.
His comments went further than the party's manifesto, which said they would keep taxes "low" but did not rule out a rise in income tax.
Campaigning in West Yorkshire, Prime Minister Theresa May declined to be drawn on his comments, saying only it was their "firm intention" to reduce taxes for ordinary families.
"Our position on tax hasn't changed. We have set it out in the manifesto," she said.
"What people will know when they go to vote on Thursday is that it is the Conservative Party that always has been, is and always will be a low-tax party.
"It is our firm intention to reduce taxes for ordinary working families."
Her comments were seized on by Mr Corbyn, who said the Conservative leadership was in disarray.
"I think there's complete chaos going on at the top of the Government," he told reporters during a campaign visit to Lincoln.
"One minister says they're going to give no more tax rises, indeed possibly tax reductions for the very wealthiest, then they can't answer the question about tax rises for the rest of the population, then they can't answer the questions about funding social care."
For the Liberal Democrats, former business minister Jo Swinson said: "The Conservatives are in utter mayhem over their tax policy."
The Conservative manifesto said there would be no increase in VAT but dropped David Cameron's pledge not to raise income tax or national insurance contributions after Chancellor Philip Hammond complained that it limited his room for manoeuvre.
However, in an interview with The Daily Telegraph, Sir Michael made it clear that income tax "absolutely" would not rise under a re-elected Tory government.
Asked if high earners could confidently vote Conservative next week, safe in the knowledge that their income tax would not go up, Sir Michael said: "Yes.
"You've seen our record. We're not in the business of punishing people for getting on, on the contrary we want people to keep more of their earnings.
"The only way they can be sure their taxes won't rise is to vote Conservative. We already know your tax will go up if you vote Labour on Thursday."
Liberal Democrat former business secretary Sir Vince Cable said that if the Conservatives were ruling out income taxes rises, they would have to find the revenue from elsewhere to meet their spending commitment.
"Michael Fallon's comments raise the obvious question as to where the Conservatives will raise the money that their Chancellor knows will be needed if promised funding for schools, the NHS, the police and defence is to materialise.
"Since they are ruling out increases in income, corporate tax and VAT, we must assume that there will be an increase in national insurance and in various 'stealth taxes' yet to be specified. It undoubtedly raises suspicions."
Former Ukip leader Nigel Farage, campaigning in Kent, said the Tories had lost control of the campaign.
"They've got different people sounding off, whether it's on immigration, which is David Davis, whether it's on tax, which is Sir Michael Fallon," he told the Press Association.
"One thing is clear, that Theresa May's government will put taxes up.
"Since 2010, since we had a Conservative Prime Minister, the tax burden has gone up."
Later, Mr Corbyn adopted a Tory attack line, accusing Mrs May’s party of having a “magic money tree” it uses to pay for tax “giveaways” to the rich.
At a raucous rally in the Nottinghamshire seat of Broxtowe, which Tory Anna Soubry held in 2015 with a majority of 4,287, Mr Corbyn mocked the slogan Mrs May has used to criticise Labour’s spending plans.
Amid chants of “Jez we can” from the crowd of around 300 at Beeston Youth and Community Centre, with a further 100 or so waiting outside, the Labour leader said: “This Government says to me ‘Mr Corbyn you’ve got a magic money tree’.
“Well I’m not sure what a magic money tree is, but I think I know one when I see one - I’ll tell you what it is, that magic money tree, when Tory chancellor after Tory chancellor gives tax giveaways to the 5%.”
Earlier, Mr Corbyn gave a stump speech to around 150 in Hucknall, in the Sherwood seat held by Tory Mark Spencer with a 4,647 majority in 2015, and mocked the PM’s “strong and stable” slogan.
“When the Prime Minister decided to surprise everybody by going walking in Wales and had an epiphany moment and decided to call a General Election, all the experts said well that’s it - strong stable government, strong stable leadership, strong stable manifesto, strong stable policies, strong stable everything.
“Well you know what? They underestimated the other things didn’t they?”