The Budget will take into account “the harsh light of fiscal reality”, but “better days lie ahead”, Sir Keir Starmer will say in a speech ahead of Wednesday's announcement.
The Prime Minister is expected to warn of “unprecedented” economic challenges, but will say the Government will “run towards them” as Labor prepares its first budget in almost 15 years.
The government is expected to announce a series of tax rises, including an increase in the National Insurance (NIC) rate paid by employers, which some say breaks Labor's manifesto pledge not to raise taxes “workers”.
The Conservatives accused Sir Keir of leading a government with “broken promises”.
In a speech in the West Midlands on Monday, Sir Keir will say the country faces an “unprecedented” challenge linked to weak public finances and a “collapse of public services”.
He will promise to face what he calls “the tough decisions.”
Speculation is growing over the tax rises the Chancellor will announce on Wednesday, with Rachel Reeves claiming there is a £22bn “hole” in the public finances left by the previous government.
Last week, Reeves signaled that businesses would face an increase in National Insurance, when she referred to Labour's election pledge not to increase “workers'” contributions linked to the personal element, as opposed to the amount paid by employers.
As well as national insurance for employers, the freeze on income tax thresholds could be extended.
This means that more people are “driven” to pay taxes or pay a higher rate and wages rise and cross the thresholds.
The government is also considering raising taxes on sales of assets, such as stocks and real estate, as well as changing its self-imposed rules on how its debts are measured to to free up money for spending on infrastructure projects.
