Boris Johnson will push Nato allies to spend more on defence while breaching a key pledge to boost funding for the military at home.

The Prime Minister is under pressure from Defence Secretary Ben Wallace and Foreign Secretary Liz Truss to boost spending in response to the renewed threat posed by Russia following the invasion of Ukraine.

The Prime Minister appeared to admit he would breach a Tory manifesto commitment for annual above-inflation rises in defence spending, but stressed that the Government was pumping in billions of pounds as part of the biggest defence settlement since the Cold War.

The Prime Minister said “you don’t look at inflation as a single data point”, insisting that over the course of the Parliament the manifesto promise would be met.

But with inflation set to hit 11% this year and the public finances battered by the impact of the pandemic, the 2019 election manifesto promise for a real-terms rise every year appeared to have been abandoned.

Mr Johnson defended his approach to the manifesto pledge: “We have been running way ahead of that target for a while now.

“We are confident that we will meet that, you don’t look at inflation as a single data point, you look at it over the life of the Parliament and I’m confident we will meet that.”

Nato has a commitment that members should spend at least 2% of gross domestic product (GDP) – a measure of the size of the economy – on defence.

The Nato summit, which starts with a welcome dinner on Tuesday night before the working sessions begin on Wednesday, comes with the organisation increasing its ability to respond to instability caused by Vladimir Putin’s actions.

Mr Johnson told reporters on the plane to Madrid: “Last year we were the third biggest defence spender in the world, we’ve another £24 billion going in under the current spending review, the biggest since the end of the Cold War. We are currently running at 2.3% of our GDP going on defence.”

Nato’s own assessment published this week indicated that the UK spent an estimated 2.26% of GDP on defence in 2021 and was on course for 2.12% in 2022.

The Prime Minister hinted he would push for a more ambitious defence spending target among Nato allies.

The Nato 2% commitment has so far only been met by nine members of the alliance.

“I think that we will have to have a conversation at Nato about where we go next,” Mr Johnson told reporters.

“And then that’s something that we’ll be talking about to friends and colleagues.”

Shadow defence secretary John Healey said: “The Prime Minister keeps breaking his defence pledges to the British public.

“With threats increasing and rising Russian threats, ministers must reboot defence plans and halt army cuts now.”

As Mr Johnson flew to Spain, the Defence Secretary made it clear he felt the need for greater investment in the armed forces.

Allies of Mr Wallace have not denied reports he submitted a formal letter to Boris Johnson calling for a 20% increase in defence spending.

In a speech at a conference organised by the Royal United Services Institute he said: “For too long defence has lived on a diet of smoke and mirrors, hollowed-out formations and fantasy savings when in the last few years threats from states have started to increase.

“Right now Russia is the most direct and pressing threat to Europe, to our allies and these shores.

“I am serious when I say there is a very real danger Russia will lash out against wider Europe. In these days of long-range missiles and stealth, distance is no protection.

“It is now time to signal that the peace dividend is over and investment needs to continue to grow before it becomes too late to address the resurgent threat and the lessons learned in Ukraine. It is time to mobilise, be ready and be relevant.”

Asked if she agreed with Mr Wallace’s concerns about the West’s ability to continue supplying arms to Ukraine and his call for a 20% boost in UK defence spending, Foreign Secretary Ms Truss told MPs: “The free world did not spend enough on defence post the Cold War and we are now paying the consequences.”

She added: “I support the aims of increasing defence spending through this Nato summit.”