Boris Johnson has announced plans to increase defence spending by the end of the decade - just days after ditching a manifesto pledge to put it up every year.
The prime minister said the UK will be spending 2.5 per cent of gross domestic product on defence by 2030.
At the moment, the government is committed to hitting the Nato target of at least 2 per cent.
Defence secretary Ben Wallace has called on the Treasury to increase defence spending from 2024.
But earlier this week it emerged that the PM had scrapped a promise in the last Tory election manifesto to increase spending by 0.5 per cent more than inflation every year.
Speaking at the end of the Nato summit in Madrid, Johnson would only commit to increasing the amount the UK spends on defence by the end of the decade.
The PM - who earlier confirmed the UK was giving another £1bn-worth of military support to Ukraine - said: “Countries around the table are also recognising that they must spend more.
“And in our case that means meeting, and being prepared to exceed, the target we set for ourselves a decade ago of everybody spending 2 per cent of our GDP on defence, goals which were then set for a very different era.
“What we are saying is that we want Jens Stoltenberg, the [Nato] general secretary, to start work on that new target now and he has agreed to do that.
“We need to invest for the long term, in vital capabilities like future combat air, while simultaneously adapting to a more dangerous and competitive world. The logical conclusion of the investments we propose to embark, of these decisions, is 2.5 per cent of GDP on defence by the end of the decade.”
Johnson also rejected suggestions that the British people would grow tired of supporting Ukraine as the war with Russia contributes to the cost of living crisis.
He said: “The point I would make about the cost of freedom, as it were, is actually it’s always worth paying.
“Unless we get the right result in Ukraine, Putin will be in a position to commit further acts of aggression against other parts of the former Soviet Union, more or less with impunity, that will drive further global uncertainty, further oil shocks, further panics and more economic distress for the whole world.”