The keynote address represents Sir Keir’s boldest attempt yet to get ahead of both the Tory leadership contenders on the question of who would run the economy best.
It will be seen as an especially pointed attack on Ms Truss, the frontrunner in the race, who has pledged over £30 billion of tax cuts including reversing the National Insurance rise.
Both the Foreign Secretary and Mr Sunak, who are set to clash in their first head-to-head TV debate tonight, have portrayed themselves as natural heirs to the Iron Lady.
“You will see a clear contrast between my Labour Party and the Thatcherite cosplay on display tonight,” Sir Keir is expected to say during his speech this morning.
“The difference between a Labour party ready to take Britain forward and a Tory party that wants to take us back into the past.
“Between Labour growth and Tory stagnation. That will be the choice at the next election and we are ready.”
He will say that, under his leadership, Labour will “care as much about growth and productivity as we have done about redistribution and investment in the past”.
Pledge not to ‘hark back’ to old ideas
And in a clear break with economic policy under Jeremy Corbyn, he will call on the party “not to hark back to our old ideas in the face of new challenges”.
Sir Keir will acknowledge this approach is set to upset those on the left of Labour but will say he is ready to face down rebellious MPs and “challenge my party’s instincts”.
“We will not be distracted by the siren calls – from the right or the left – that say economic growth and net-zero do not go together,” he will add.
Ahead of the debate, Labour has branded both candidates “stooges of the Johnson administration” and published five questions it says both should be forced to answer.
It says they should spell out if they would fund tax cuts by borrowing more or reducing spending on public services and whether they have ever used offshore trusts to avoid tax.
Sir Keir also wants the pair to say whether they would scrap non-dom status and tax relief for private schools and what they would do to help Brits with their energy bills this winter.