Theresa May has ruled out increasing VAT for the next five years but has refused to reveal whether she will increase other taxes for higher earners.
The Conservatives are understood to be considering an election pledge not to increase the total tax take - as a proportion of the economy - but are poised to abandon David Cameron's commitment not to increase income tax and national insurance.
Philip Hammond was recently forced to abandon plans to increase National Insurance for the self-employed as it risked breaking the 2015 Tory election manifesto, but a rewritten tax pledge would allow such a rise.
The Prime Minister has repeatedly refused to speak about her tax plans in detail since launching the election campaign last month but yesterday ruled out any rise in VAT. She refused to give a similar pledge on income taxes.
Mrs May pledged to target tax cuts at "working families" and may also introduce new tax breaks for those saving to cover the cost of social-care.
In an interview with ITV’s Peston on Sunday programme, she said: "“We have no plans to raise the level of tax. In relation to specific taxes, we won't be increasing VAT.”
She used a similar form of words in another television interview in which she said: "“We have absolutely no plans to increase the level of tax but I'm also very clear that we don't want to make specific proposals on taxes unless I'm absolutely sure that I can deliver on those.
“But it would be my intention as a Conservative government and as a Conservative prime minister to reduce the taxes on working families. And if you’ve got strong and stable leadership that's absolutely what you can do.”
There was speculation - which was strongly denied by Mrs May's aides - that capital gains tax might be levied on sales of main homes worth over £5million.
State pensions would also continue to rise over the next five years but there was no commitment to maintain the triple lock which ensures the state pension increases in line with wages, inflation or by 2.5 per cent - whichever is the highest, she said.
She said: “Under a Conservative government the state pension will still go up every year of the next parliament. Exactly how we calculate that increase will be for the manifesto, and as I have just said you will have to wait for the manifesto to see what’s in it.”
Mark Littlewood, director general of the Thatcherite thinktank the Institute for Economic Affairs, said Mrs May’s “wording suggests that taxes will probably go up if she is re-elected”.
He said: “Claiming to have no plans to raise taxes is standard politicians' code for increasing tax due to supposedly unforeseen future circumstances.
“She may be justified in not to making ultra-specific pledges on individual taxes, although she has done so with VAT.”
The Government’s tax take as a proportion of national income was the same in 2015 – 32.5 per cent – as it was when the Conservatives first came to power in 2010.
Mr Littlewood added: “There is no reason why she could not commit to the total tax burden falling as a percentage of overall national income if the economy continues to grow.”
Dia Chakravarty, political director at the TaxPayers' Alliance, added: “It is deeply concerning that Mrs May is not ruling out the possibility of tax rises on an already highly taxed population.
“A promise to cap tax burden at 35 per cent of GDP from Mrs May would signal an intention to reduce the burden while allowing the flexibility on the profile of the tax take.”