The party has accused the previous Conservative government of leaving a £22bn “black hole” of unfunded spending commitments in the public finances – something the Tories have disputed.
Sir Keir will repeat the claim in his speech, saying the Tories have also “decimated public services, leaving communities held together by little more than goodwill”.
But he will warn the pockets of working people “are not deep” and public services will need “reform” as well as investment.
“Just because we all want low taxes and good public services, does not mean that the iron law of properly funding policies can be ignored,” he will add.
Labour has repeatedly promised it will not raise taxes on “working people”, including VAT, National Insurance and income tax.
However, the chancellor has already said she will have to raise some other taxes in October’s Budget because of the state of the public finances.
The government has also blamed the £22bn black hole for the decision to cut winter fuel payments.
A debate and non-binding vote on the issue had been expected on Monday but could now take place on Wednesday, when many activists will have left the conference.
Unite, which is among the trade unions calling for the cut to be reversed, branded the delay an “outrage”.
