Landlords and shareholders face being hit with tax rises at next week's Budget after Sir Keir Starmer suggested they are not 'working people'.
The PM fuelled alarm at the looming raids by Rachel Reeves as he suggested those with assets will by targeted by Labour.
The Chancellor is expected to impose the biggest tax hikes in three decades on Wednesday, raising around £35billion extra to take the burden on Brits to a record high.
But Sir Keir has been under pressure to set out what a 'working person' is after Labour's manifesto said they would be shielded from increases.
He has already ruled out putting up rates of income tax, employee National Insurance and VAT, leaving Ms Reeves scrambling to target areas such as capital gains and inheritance tax for revenue.
In an interview with Sky News at the Commonwealth summit in Samoa, Sir Keir said a working person is somebody who 'goes out and earns their living, usually paid in a sort of monthly cheque' but they did not have the ability to 'write a cheque to get out of difficulties'.
And when asked if this would include people who get all or part of their income from assets, he said: 'Well, they wouldn't come within my definition.'
Downing Street later tried to quell a backlash at the premier's remarks, insisting people who hold a small amount of savings in stocks and shares still count as 'working'.
The PM's official spokesman said Sir Keir had been referring to Brits who primarily get their income from assets.
Sir Keir's comments come amid widespread expectation that the Chancellor will increase capital gains tax on profits from the selling of shares.
She is also understood to be planning to impose national insurance on employers' contributions to retirement funds - despite complaints it is a 'straightforward breach' of Labour's manifesto.
The rumoured £35billion of tax increases in the package would be the most raised at a Budget since 1993.
It would take the tax burden to a new peak since comparable records began in 1948 - and it is not thought to have been higher before that.
However, Sir Keir has insisted there is 'no reason' for entrepreneurs to leave the country.
He said: 'My evidence that what we are saying is attractive to investors is last Monday's investment summit that was hugely successful.
'All the feedback back to us has been that it was very well received by a significant number of global investors.'
Sir Keir insisted people were investing in Britain 'because of what this government is bringing to the table'.
Downing Street later said investors 'shouldn't be worried about this Budget', despite some rushing to sell assets due to expected hikes in capital gains tax.
Touring broadcast studios this morning, Treasury minister James Murray told Sky News that 'a working person is someone who goes out to work and who gets their income from work'.
Pushed further on whether a working person could also get income from shares or property, Mr Murray added: 'We're talking about where people get their money from, and so working people get their money from going out to work.
'And it's that money that we're talking about in terms of those commitments we made around income tax, around national insurance.
'That's what's important to focus on, where people are getting their money from, getting their money from going out to work.'
Sir Keir has said the Budget will aim to 'fix the foundations' and 'rebuild' the country as he insisted that the '£22billion black hole' is 'for real' and not 'performative'.
'It's for real and we've got to deal with it and I don't think we are wrong to be honest about that and we have also been clear this is a budget about rebuilding the country and therefore it will also spell out the direction of travel for the country and what we want to do with it.
'We've got to get both bits of that right.'
The PM said he was 'not prepared' to put off the pain for another year, telling reporters that while there would be more budgets to come, he wanted to 'tackle the inheritance in this Budget'.
'I'm not prepared to walk past it. I'm not prepared to put it off and that is a signal of the way I want to do business which is not to pretend our problems aren't there, it's to actually roll up our sleeves and deal with it.'