Rachel Reeves will use her first Spring Statement as chancellor to announce a £2.2 billion boost in defence spending.
The extra funding, which will be partly paid for by cuts to international aid, will be spent on new hi-tech weapons and better homes for military families.
Reeves will say that the extra military spending is needed to “secure Britain’s future in a world that is changing before our eyes”.
However, the chancellor is also expected to announce deep spending cuts in other areas as the Office for Budget Responsibility confirms the £10bn in spare cash she expected to have at the Budget last autumn has disappeared.
The OBR will also downgrade its forecast for UK economic growth this year from 2% to 1% in a further blow for Reeves.
Despite the gloomy backdrop, she will say she is “proud of what we have delivered in just nine months”.
“Now our task is to secure Britain’s future in a world that is changing before our eyes,” the chancellor will tell MPs.
“The job of a responsible government is not simply to watch this change.
“This moment demands an active government stepping up to secure Britain’s future. A government on the side of working people.
“To grasp the opportunities that we now have and help Britain reach its full potential, we need to go further and faster to kickstart growth, protect national security and make people better off through our plan for change.”
Keir Starmer announced last month that the defence budget will increase from 2.3% to 2.5% of gross domestic product (GDP) from 2027, before hitting 3% after the next election.
Reeves will say: “That was the right decision in a more insecure world, putting an extra £6.4bn into the defence budget by 2027.
“But we have to move quickly in a changing word. And that starts with investment.
“So I can today confirm that I will provide an additional £2.2bn for the Ministry of Defence next year - a further downpayment on our plans to deliver 2.5% of GDP.
“This increase in investment is not just about increasing our national security but increasing our economic security, too. As defence spending rises, I want the whole country to feel the benefits.”
Some of the extra money will be used to fit Royal Navy ships with so-called “directed energy weapons”, which can hit a £1 coin from 1km away and take down drones at a distance of 5km.
Funding will also be made available to refurbish more than 36,000 homes recently brought back into public ownership from the rental sector to be used by military families
Money will also be spent on upgrading His Majesty’s Naval Base Portsmouth.
Defence secretary John Healey said: “This investment will make Britain stronger and safer in a more insecure world. And it will ensure defence is an engine for growth, creating good jobs across the nation.
“These are the bold first steps of the largest sustained increase in defence spending since the Cold War.
“Our government is delivering for defence and investing in the outstanding men and women who keep Britain secure at home and strong abroad.”