It's the first budget by a Labour Party government in almost 15 years, and the first ever delivered by a female finance minister.
Reeves pledged Wednesday's budget would put "more pounds in people's pockets" and get the economy growing, but the government has struck a gloomy note about the state of the public finances.
Prime Minister Keir Starmer warned the budget would reflect "the harsh light of fiscal reality".
The centre-left Labour Party was elected July 4 after promising to banish years of turmoil and scandal under Conservative governments, get Britain's economy growing and restore frayed public services, especially the state-funded National Health Service.
The government argues that higher taxes and limited public spending increases are needed to "fix the foundations" of an economy that it argues has been undermined by 14 years of Conservative government.
The Conservatives say they left an economy that was growing, albeit modestly, with lower levels of debt and a smaller deficit than many other G7 nations.
Pumping money into health, education and housing is a priority of the new government, made harder by a sluggish economy, hobbled by rising public debt and low growth.
The government also says there is a Stg22 billion ($A43 billion) "black hole" in the public finances left by the Conservative government.
That means the budget is certain to include tax increases, though Labour has pledged not to raise the tax burden on "working people", a term whose definition has been hotly debated in the media for weeks.
The Treasury has announced that about three million of the lowest-paid workers would get a 6.7 per cent pay increase in 2025, with he minimum wage rising to Stg12.21 an hour.
Reeves - Britain's first female chancellor of the exchequer - is widely expected to tweak the government's debt rules so that she can borrow billions more for investment in the health system, schools, railways and other big infrastructure projects, and to raise money by hiking tax paid by employers, though not employees.
