Rachel Reeves will promise to “secure Britain’s future” by boosting defence spending in Wednesday’s spring statement, as she faces mounting speculation that she will be forced to raise taxes in the autumn.

Speaking to MPs on Wednesday as she responds to the latest forecasts from the Office for Budget Responsibility (OBR), the chancellor will announce that she has set aside an extra £2.2bn for defence next year.

The additional funding is a down payment against the government’s target of spending 2.5% of GDP on defence – paid for by cutting aid spending and dipping into the Treasury reserve.

The chancellor will reiterate the government’s “ambition” to spend 3% of GDP on defence in the next parliament, “as economic and fiscal conditions allow”.

Reeves will confirm the £5bn in welfare cuts announced last week, and publish impact assessments that will reveal how individuals could be hit.

She is also expected to squeeze future Whitehall spending plans, to ensure she is on target to meet her self-imposed fiscal rules, despite weaker OBR projections – with full details to be set out in June’s spending review.

Since the OBR last gave its assessment in October, government borrowing costs have risen and economic growth has been weaker than hoped.

Reeves will underscore her determination to go “further and faster” to boost economic growth, with the OBR expected to have revised down its GDP forecast in the short term because of weaker than expected data at the end of last year.

Some in Labour had urged Reeves to flex her fiscal rules instead of outlining future spending cuts – but the Treasury fears that any sign of indiscipline would risk spooking bond markets and driving borrowing costs up further.

With Donald Trump’s administration withdrawing from transatlantic defence cooperation and threatening to impose sweeping tariffs next month, Reeves will repeatedly stress how much the global context has changed.

“Our task is to secure Britain’s future in a world that is changing before our eyes. The job of a responsible government is not simply to watch this change,” she will say.

But analysts warn that these historic shifts mean that even after promising spending cuts, Reeves may still have to increase taxes to meet rapidly growing pressure for higher defence spending.

Paul Johnson, director of the Institute for Fiscal Studies, said: “Even a small change to the spending plans is going to make this an even more difficult spending review in June, and I think the bigger risk is that we get speculation starting on Thursday about which taxes are going to rise in the autumn – and I think that is really quite politically risky, and economically damaging.”

Paul Dales, chief UK economist at consultancy Capital Economics, said: “We just don’t know how they intend to increase spending above 2.5% – that’s the really big one. That’s what’s changed.” He added: “The thing that’s really going to have to shift is her pledges on tax.”

Prof Jonathan Portes, of King’s College London, said: “I don’t think they should be making big policy changes now, but I do think over time they will have to reform and increase taxes.”

Reeves will tell MPs the move to boost defence spending, which saw development minister Anneliese Dodds resign in protest at the aid cuts, was “the right decision in a more insecure world”.

“This government was elected to change our country. To provide security for working people. And deliver a decade of national renewal. That work of change began in July – and I am proud of what we have delivered in just nine months,” she will say.

The chancellor and her Treasury team have been trying to limit the likely fallout from Wednesday’s announcement, aware that both the cuts to Whitehall departments and the welfare impact assessments are likely to cause anger on the Labour benches.

One Labour MP said: “Wednesday will be just as important for those impact assessments as for the spring statement itself – that’s when people will start to make their mind up about whether they will vote for these cuts or not.”

Officials say they are planning to hold a vote on the changes to personal independence payments in May, with about 30 Labour MPs currently thinking of rebelling.

Darren Jones, the Treasury chief secretary, held a meeting with about 100 frontbenchers on Tuesday to lay the ground for the spending cuts to come. People who attended that meeting told the Guardian he had spent much of it insisting that the spending reductions did not amount to austerity, given they are around half the scale of those made by George Osborne as chancellor from 2010 to 2015.

Wes Streeting, the health secretary, told the Guardian on Tuesday: “We can’t do everything for everyone, everywhere, all at once. There are lots of things we would like to do now, but we’re having to bide our time so that we can fix the economy, because having those firm economic foundations is the essential prerequisite for everything else we need.”

Reeves will announce details of a government transformation fund that Whitehall departments will be able to bid into, to pay for productivity-boosting projects such as overhauling out-of-date IT. Treasury ministers claim this will allow them to do more with less in future years, easing the impact of tighter budgets on public services.

Reeves will confirm on Wednesday she will start moving money from the aid budget to defence immediately, dashing the hopes of some Labour MPs who hoped the cuts to the development budget would be delayed until 2027.

Sarah Champion, the Labour chair of the international development committee, said: “The government’s statement on cutting aid has had a chilling effect on development projects and staff morale, but it has also had very real consequences. Contract renewals are paused and new projects on hold. Whichever way to pack it, cuts are happening now.”

The £2.2bn boost in defence spending from April will bring the country’s military spending up from 2.3% in 2024-25 to 2.36% in 2025-26. Ministers have promised to hit the 2.5% target in two years’ time.

Part of the additional defence money will be spent guaranteeing an investment to fit navy ships with Dragonfire lasers – weapons which can hit high-speed missiles and drones from a mile away. The Conservatives first announced the plan to fit the lasers to navy ships a year ago, but Labour sources said they had not allocated the funding to ensure it happened.

Labour hopes to use the significant increase in defence spending in the coming years to create jobs across the UK. Reeves will say on Wednesday: “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.”

The shadow chancellor, Mel Stride, said Reeves, not global events, was to blame for the slowdown in the economy. “Our national security demands a strong economy. Yet since Rachel Reeves’s first budget, growth is down, borrowing is up and business confidence has been destroyed,” he said.

Not all analysts are pessimistic about the prospects of the chancellor sticking to her tax and spending pledges.

Andrew Wishart, of Berenberg bank, said: “Savings on welfare could be higher than expected. Meanwhile, pay growth is making a habit of coming in above the OBR’s prediction which will boost future tax receipts. That could allow the government to top up departments spending in the years ahead without resorting to tax hikes.”

Additional reporting Aletha Adu