Rishi Sunak has claimed Liz Truss's tax cuts would benefit "very large companies" while overlooking the most needy as the energy price crisis deepens.
The former chancellor is playing catch-up with frontrunner Ms Truss amid increasingly angry blue-on-blue clashes following terrifying forecasts that the energy price cap could top £4,200 in January.
Mr Sunak told the BBC's Nick Robinson that Ms Truss's tax cutting measures would fail to support "the millions of people who don't pay enough tax" - adding his winter payment plans would target those "that most need our help".
He suggested that Ms Truss's tax-cutting proposals were not "the moral thing to do".
Mr Sunak said: "The tax cuts that Liz Truss is proposing largely benefit very large companies."
He also claimed he would rather lose his dream of becoming PM to "do the right thing" - despite being repeatedly criticised for lurching to the right in desperate efforts to impress the party faithful.
The former chancellor, whose resignation helped spark the crisis that brought down shamed Boris Johnson, is trailing Ms Truss - who herself admitted her proposed measures could take months to come into effect - in polls.
Ms Truss has repeatedly spoken out against giving "handouts" to target struggling families, instead accusing Mr Sunak of "Gordon Brown style economics".
In response Mr Sunak said: "I do feel a moral responsibility as prime minister to go further and get extra help to people over the autumn and the winter to help them cope with what is going to be a really difficult time.
"I think that is the right priority."
Asked whether the extra payments would cost a "few billion" or over £10 billion, he replied: "It's much closer to the former than the latter."
He admitted that the unfolding situation is far worse than he expected when he first announced support measures, stating: "In order of magnitude it looks like the situation is around £400 worse give or take than we thought when I announced support earlier this year.
"That gives you a sense of the scale of what we are talking about extra and then we have to think about how do we split that up, what do we do for the most vulnerable people on the lowest incomes, what do we do for pensioners, and what do we do for hardworking families?"
He later suggested total support for families could be lifted to "£700 to £800" through a cut to VAT on energy bills, on top of existing support.
Mr Sunak said: "I do think there has been a breakdown in trust because of the issues of the last several months and part of why I am sitting here is because I want to restore trust back into politics.
"I want to put integrity and honesty at the heart of how I run government and how I want to be prime minister. And as you can see in this leadership contest, I have been doing that.
"I haven't been saying the easy things and actually I am prepared to lose this contest if it means that I have been true to my values and I am fighting for the things that I think are right for this country.
"I would rather lose on those terms than win by promising false things that I can't deliver."
Floundering Ms Truss was today told "we need help now" by a dad forced to get a second job in order to pay bills - as she admitted her tax cuts could take months to enact.
Faced with terrifying rises in the energy price cap, which forecasters estimate could top £4,200 in January, Ms Truss faces desperate calls for action if she becomes PM.
She claimed she would act on "day one", but was forced to admit that her proposed tax cuts might not take effect until spring next year.
Confronted by voter Neil Darwish during The People’s Forum With Liz Truss broadcast by GB News, the frontrunner continued to claim that reversing a rise in National Insurance and pausing green energy levy would ease the pain for families.
She said that she would begin the process "on day one" of her premiership, but was forced to concede that it could still take until spring to bring her measures into effect.
"I will act on day one but it will come in well before April, which is when traditionally the tax year starts," the flustered Tory candidate stated, before adding: "But I will make sure the Chancellor does it as soon as he can."
Repeating her go-to line on handling the crisis, Ms Truss said: "My fundamental approach is we need to keep taxes low. What we shouldn't be doing is taking money from people in taxes and giving it back to them in benefits."
But Mr Darwish, a manager in the social housing sector, told her: "I need money now, we need money now, we can't wait."
The frontrunner told presenter Alistair Stewart: "There would be an emergency budget under my budget that would immediately look at these issues."
Ms Truss repeatedly failed to answer questions over whether a government with her at the helm would introduce a bailout scheme to help struggling families, instead stating: "We don't yet have a new Prime Minister, that happens in September, so I can't announce what I'll do at this stage."
However, she added: "If the only answer to everything is to whack up taxes and give out more benefits then the country is going to run into trouble.”
Earlier, she denied she was ruling out direct support for families - sparking claims of a new U-turn.
It follows an astonishing day of blue-on-blue attacks.
Mr Sunak told the BBC he would rather lose than "win on a false promise", saying the next PM has a "moral responsibility" to support the most needy.
He said he would stay "true" to his values.
The former chancellor said the next PM has a "moral responsibility" to support poorer households with payments for energy bills.
