Keir Starmer has hinted at tax rises for those who earn their income from shares and property, saying that they did not fit his definition of “working people”.

Ministers are expected to announce increases in inheritance tax and capital gains tax (CGT) in the budget next week.

Ahead of the anticipated tax rises, the government has come under pressure to clarify its manifesto promise to “not increase taxes on working people”.

Asked whether someone who works but also has an income from assets, such as shares and property, was a working person, Starmer told Sky News: “Well, they wouldn’t come within my definition.”

The prime minister’s spokesperson later clarified that he was referring to people who “primarily get their income from assets” and was “not precluding people that have a small amount of savings” in stocks and shares.

Pressed on whether that meant taxes for these people could go up, Starmer told Sky News: “You can probably give me any number of examples … you’re asking me for a definition of who’s a working person, and then you’re making assumptions about what that tax might be in relation to.”

The debate over the definition of working people has intensified after ministers refused to rule out raising national insurance on employers in the budget.

Starmer was asked to set out his definition of working people by reporters travelling with him for the Commonwealth summit in Samoa: “I have in mind people who go out to earn their living, may have some savings, but don’t have the ability to sort of routinely write a big cheque if they get into difficulties.

“They’re the people uppermost in my mind when we’re making our decisions,” he added.

Starmer and Rachel Reeves, the chancellor, have both warned that they will need to make “tough decisions” in the budget.

Starmer has indicated that CGT on the sale of shares and other assets, currently set at up to 20%, will increase. The tax is expected to rise by several percentage points.

However, ministers are expected to leave CGT on the sale of property untouched because of concerns that increasing it would cost money by slowing down sales. The Conservatives cut the top rate of CGT for property from 28% to 24% in the last budget.

Reeves is also looking at tightening the rules for inheritance and gift tax. Only about one in 20 UK estates now attract inheritance tax.

On Friday, James Murray, a Treasury minister, refused to be drawn on whether a landlord would be considered a “working person” for the purposes of Labour’s manifesto pledge not to increase tax on “working people”.

“Working people are people who go out to work for their income,” he told BBC Radio 4’s Today programme. “We were very clear in our manifesto.”

Asked repeatedly if a landlord was a working person, after Starmer said a person whose income derived from assets, such as shares or property would not “come within my definition” of a working person, Murray said: “We’re talking about where people get their income.”

Murray insisted any changes to fiscal rules would be in line with what the Labour party said in its manifesto.