Keir Starmer has confirmed drastic cuts to Britain’s international aid budget to help pay for a major increase in defence spending as European nations attempt to fill the gap left by Donald Trump on Ukraine.

The prime minister confirmed the UK government’s timeline to increase defence spending to at least 2.5% of GDP by 2027 – three years earlier than planned – as he prepares for what is likely to be a diplomatically fraught visit to Washington DC.

Starmer also announced an ambition to increase defence spending to 3% during the next parliament. However, he confirmed that the rise by 2027 would be paid for by a “painful choice” to cut the aid budget, from 0.5% of GDP to 0.3%.

He told MPs: “We will do everything we can to return to a world where that is not the case, and rebuild a capability on development, but at times like this the defence and security of the British people must always come first.

“That is the number one priority of this government.”

The Guardian first reported the prime minister’s plan to boost UK defence spending, funded by aid cuts, in the face of Russian aggression moments before he took to his feet in the House of Commons on Tuesday. He immediately announced a leak inquiry.

Starmer has come under continued pressure to rapidly lift defence spending even further than Labour’s manifesto pledge of a rise to 2.5% by 2030, after he said the UK would “play its full part” in deploying troops to Ukraine for a peacekeeping force in the event of a durable deal after Russia’s invasion.

Defence sources have said that an increase of at least £13.4bn a year to 2.5%, from 2.3%, would still be far short of what is required to rebuild and transform the armed forces.

They have stressed that an ultimate rise to at least 3% of national income would be necessary in order to help boost military capability after Trump’s administration said it was scaling back US support from Ukraine.

Labour had promised to raise the aid budget from 0.5% to 0.7% when “fiscal conditions allow”, but in recent weeks officials have begun to look at cutting it instead as a way to help pay for the planned increase in defence spending.

David Lammy, the foreign secretary, told the Guardian earlier this month that Trump’s plans to make dramatic cuts to the US’s international aid budget could be a “big strategic mistake” that would allow China to step in and extend its global influence.

Simon McDonald, the former head of the Foreign Office, has said such a move would damage Britain’s global reputation.