Keir Starmer has called on Rishi Sunak to boost defence spending in his Spring statement this month.
The Labour leader told the Times the Chancellor should use the statement to boost cash for Britain's armed forces.
“I do think that the government needs to use the [statement] now in two weeks’ time as an opportunity to look at our defence spending,” he said.
“I think a lot of his own side will be looking at him to do that.”
Earlier this week, Foreign Secretary Liz Truss indicated she too would favour increasing spending on defence.
Speaking to CNN in the US, she said: "We went through a period after the Cold War where we all thought that peace would be inevitable, that we could reduce defence spending, that we could focus on growing our economies. We became dependent on authoritarian regimes."
She went on: "If you look at Europe and our dependence on Russian oil and gas, that has been a huge problem. So, what we need to do now is completely change our approach.
"That's why we put in place (these) tough sanctions, and today I've announced that we're sanctioning Roman Abramovich as well as other major oligarchs close to the Putin regime.
"We need to take this head on, (there) can be nowhere to hide for these individuals. We cannot carry on with business as usual, as we have been doing for the past 20 years."
And former Foreign Secretary Jeremy Hunt today called for UK spending to match the US.
Mr Hunt said the war in Ukraine was "the biggest failure of Western foreign and security policy in our lifetimes".
And he suggested the UK should increase spending on defence, aid and soft power to at least 4% of GDP over the next decade.
In 2021, the UK spent 2.3% of GDP on defence - around £40 billion - compared to 3.5% for the USA.
