Andy Burnham has hinted more tax hikes could be on the table as the strain on the public finances hits home.
The PM admitted the country is in a 'challenging position' amid claims he has delayed plans to put Thames Water into administration due to the eye-watering costs.
In a series of interviews on a visit to Ukraine, Mr Burnham suggested he had to be 'realistic' about how much help he could give struggling families.
The comments have renewed fears that another raid is looming in the Budget on October 28 - although Mr Burnham repeated that he would stick to Labour's manifesto promises not to increase income tax, VAT or national insurance.
Left-wingers have been ramping up pressure for so-called 'wealth taxes', despite alarm at the potential impact on the economy.
Mr Burnham has also previously backed a 'land value tax' that critics warn would cause havoc in London and the South East.
In a series of interviews on a visit to Ukraine, Andy Burnham suggested he had to be 'realistic' about how much help he could give struggling families
The premier again denied he would call a snap general election to get a new mandate, despite signs of a 'Burnham bounce' in the polls.
Mr Burnham has made clear he would prefer to see the water system in public hands, with suggestions that Thames Water could be put into a special administration regime (SAR).
That would allow essential services to be maintained until a buyer was found for the debt-laden company - which serves 16million customers.
However, the Times said officials had raised concerns that the firm would need £2billion of funding in the coming year alone.
'It is very challenging and there are significant legal risks,' one Whitehall source said.
Downing Street insisted an SAR was still 'on the table', stressing that the move was never going to be 'immediate' and the situation is 'complicated'.
In Kyiv yesterday, Mr Burnham was asked by ITV News if the public had to accept paying more in tax to fund policies such as more council housing and social care provision.
He replied: 'No, they don't necessarily have to accept that.'
When it was put to him that the hole in the defence budget needed to be filled with tax rises, the PM said he 'won't take risks with people's jobs or their livelihoods'.
He added: 'I will try to help them in whatever way I can, I have already done some things that will help them. I will do what I can but I won't be unrealistic.
'We are in a challenging position. Whatever I do will be carefully thought through, it will be funded and there will be more to come as we go into the autumn.'
Mr Burnham has made clear he would prefer to see the water system in public hands, with suggestions that Thames Water could be put into a special administration regime (SAR)
In an interview with LBC he was again asked if the public should expect to pay more tax in the October Budget, and replied: 'We have a manifesto, and I will honour that manifesto. I believe it's possible to provide extra help for people with the cost of living by changing some of our priorities.'
He added: 'Whether I will achieve everything I want to do, that's a different matter. But people will be in no doubt about the direction in which I seek to take the country...a hopeful direction, a positive direction, a unifying one.'
Told that it would be easier to get a new mandate by calling an election, Mr Burnham replied: 'I am ruling that out right in front of you.'