Plans to limit the number of pensioners who get winter fuel payments are "sick and sneaky", Labour has claimed.
Shadow chancellor John McDonnell said 10 million people would be hit by Tory proposals to means-test the allowance.
The current universal provision was the "basis of the welfare state" and moving away from it would hit the poorest and worsen fuel poverty, he told the BBC.
The Tories say help will be focused on those most in need and the savings directed at supporting social care.
Lib Dem leader Tim Farron said that while he backed the means-testing of winter fuel allowances, other Tory policies such as ending free school lunches for infants and imposing what he characterised as a "dementia tax" for those needing care at home were "cruel and heartless".
The SNP said the Conservative manifesto "robs Peter to pay Paul - stealing free school meals to pay for education, cutting winter fuel to pay for social care".
At the moment, households automatically receive a single payment, ranging from £100 to £300, each December if there is one person living there who has reached the qualifying age and who meets other criteria.
About 12.26 million people received the tax-free allowance in 2015-6, at a total cost of just over £2bn.
Under plans in the Conservative manifesto, eligibility would be related to income - although the party has not indicated what the threshold would be and who would qualify.
But Mr McDonnell told the BBC that the scale of the money that the Conservatives hoped to raise - which he said was in the region of £1.5bn each year - suggested all those who did not qualify for pension credit could lose their allowances, hitting millions of those on low and middle incomes.
"This means 10 million pensioners waking up this morning to the fact they could lose their allowance.
"We also have 1.7 million pensioners in this country living in poverty, a million of them in fuel poverty...30,000 excess deaths every winter as a result of fuel poverty and basically people not being able to heat their homes."
Critics have argued that fuel allowances, introduced in 1997, are poorly targeted with everyone getting them regardless of their financial circumstances, including thousands living outside the UK.
In 2014, ministers encouraged better-off pensioners who could afford their heating bills to return the money to the state.
But figures obtained by the BBC found just 29 pensioners decided to decline their fuel allowance in 2014-5.
In their 2015 manifesto, Labour proposed taking fuel allowance away from the wealthiest 5% of pensioners.
But Mr McDonnell said he was "really angry" that the universal principle of welfare support for pensioners, which has been upheld by successive governments, was now being threatened.
"We have a universal benefit at the moment. Why? We know that as soon as you start means-testing, a lot of people don't claim even though they need it because it is so complicated."
Labour have said the means-testing of fuel allowances, allied to the ending of the "triple lock" guarantee for pension spending and reforms to social care rules, amounted to a "triple whammy" for pensioners.
The shadow chancellor also took issue with what he said was a total lack of detail about how the Conservatives planned to pay for up to 60 commitments in their manifesto.
"On every item in our manifesto, I rolled out a detailed costing and funding source.
"You would not let someone go off to the supermarket with a blank cheque, take things off the shelves and not tell you how much they are going to pay for it. It's ridiculous."
Brexit Secretary David Davis dismissed reports that up to 10 million pensioners might lose the winter fuel allowance as "guesswork", telling the BBC that the government would consult on means testing and the only people to lose would be "those who can afford it".
Separately, the former Conservative chancellor Ken Clarke said he was "delighted" with his party's decision not to put detailed costings into its manifesto, saying this would be a "hostage to fortune".
He told Radio 4's Today the 2015 manifesto, which committed the Conservatives to not raise VAT, income tax and national insurance for five years, had been "crazy".