"Let me be really clear - every minute that's not spent talking about and dealing with the cost of living is a minute wasted," the prime minister told the Daily Mirror.
Sir Keir said that he understood the cost of living was "the single most important issue" for Britons, and emphasised that his government aimed to make people feel "better off" amid rising costs.
"I'm very conscious of the fact that people want to get on in life, they want to progress, they want more money in their pocket to do the things that matter to them," he said.
"Every minute we waste on other issues is a minute we're not bearing down on that."
Sir Keir addressed several measures which are expected in the Budget, including a freeze on prescription charges and increased free breakfast clubs for primary schools.
Reeves will outline her Budget in the House of Commons on 26 November, which will include big decisions regarding spending on public services.
Despite strong indications from the chancellor that she would raise income tax rates, government sources have said that Reeves has decided against the approach.
The sources maintain that "tough choices" will still have to be made in the Budget, and have stressed that the move is not related to questions regarding the prime minister's future in the Labour Party.
Following the chancellor's speech, the Office for Budget Responsibility will also release its assessment of the government's plans alongside its latest forecast.
Both Conservative leader Kemi Badenoch and Reform UK leader Nigel Farage are expected to set out their respective plans for managing public finances later on Tuesday.
