Boosting defence spending is essential to tackling the 'generational threat' posed by Russia and other hostile states, Sir Keir Starmer warned.
At the end of the Nato summit in Washington, the Prime Minister suggested member states should aim to spend at least 2.5 per cent of GDP on defence.
He has come under fire at home for refusing to put a clear timetable on meeting the same target at home.
Today he said that Nato was 'stronger than ever', with 23 of its 32 members now spending 2 per cent on defence.
But he added: 'In light of the grave threats to our security, we must go further. So we will conduct a strategic defence review to strengthen our Armed Forces and protect our national security. And we will set out a clear path to spending 2.5 per cent of our GDP on defence.
At the end of the Nato summit in Washington, the Prime Minister (pictured speaking with US President Joe Biden) suggested member states should aim to spend at least 2.5 per cent of GDP on defence
Russian President Vladimir Putin is viewed as a threat by the member states of Nato
Emergency and rescue personnel along with medics and others clear the rubble of a children's hospital following a Russian missile attack in the Ukrainian capital of Kyiv on July 8
'I say with candour to all our allies we must understand that this is now essential.
'This is a defensive alliance. We do not seek conflict. But we know that the best way to avoid it is to prepare for it.' The PM said the 'generational threat from Russia, demands a generational response'.
And he warned that Russia's missile attack on a children's hospital in Kyiv this week showed 'the alternative to Ukraine's victory is unthinkable'.
'As I told President Zelensky today, Nato will be stronger with Ukraine as a member.'
Sir Keir said Nato's response to the challenge posed by states such as Russia, China, North Korea and Iran 'will shape the world for decades to come'.
President Zelensky says for Ukraine to win the war, the US needs to lift the limits on using its weapons to strike inside Russia.
The threat from Moscow was also addressed by Defence Secretary John Healey as he too insisted the new government will increase spending on the armed forces.
NATO leaders pose for a family photo before President Joe Biden, front row center, delivers remarks on the 75th anniversary of NATO on Tuesday
The threat from Moscow was also addressed by Defence Secretary John Healey (left) as he too insisted the new government will increase spending on the armed forces
The PM, making his debut on the world stage at the NATO summit in Washington DC, told the Ukrainian President there would be 'no change of approach' under his Government
The PM pictured with NATO Secretary General Jens Stoltenberg as they met in Washington DC
Mr Healey said the UK was 'facing growing threats across the board'. He told reporters at the summit that the UK could also take part in more European defence structures, but ruled out joining a European army.
Sir Keir has been dogged by questions about defence spending at this week's summit after refusing to set out a timetable for raising it to 2.5 per cent of GDP.]
Mr Healey insisted a timetable would be set out once the Government had conducted its strategic defence review.
'What is really clear is that we're facing a decade or more of growing Russian aggression,' he said.
'Supporting Ukraine and its ability to win is our first priority. And when we're facing growing threats across the board, then... the strategic defence review is our chance to get to grips with the threats that we face, the capabilities that we've got, the true state of the Armed Forces and of course the finances that may be available.
'It needs to be done at pace, it will be done properly,' he said.
Mr Healey insisted that seeking closer defence ties with Brussels would not bind the UK into a European defence force.
Mr Healey said: 'What is really clear is that we're facing a decade or more of growing Russian aggression'
Emmanuel Macron and Jill Biden were pictured holding hands at the 75th Nato summit
Security minister Dan Jarvis (pictured in 2015) said it was 'unfair' to criticise the new government for wanting to assess the state of the UK's defences before finalising a path to higher funding
But he suggested the UK could seek to join more areas of the EU's permanent structured co-operation (Pesco) programme. The UK is already part of one Pesco scheme on moving military equipment across the EU.
Security minister Dan Jarvis said it was 'unfair' to criticise the new government for wanting to assess the state of the UK's defences before finalising a path to higher funding.
He told Sky News: 'It's important that we follow the process and look carefully at the nature of the threat that our country faces, and the resources that will be required to put in place an armed forces that is fit for purpose.'
Sir Keir insisted the spending target would be hit eventually, saying: 'What we've had for the last 14 years was a series of arbitrary dates and lack of funding for meeting those targets which were missed over and over again.'
Emmanuel Macron and Jill Biden were pictured holding hands at the 75th Nato summit. It comes as the alliance debates how to 'Trump-proof' its future and its support for Ukraine in case former US president Donald Trump is returned to the White House in November.
Mr Trump has criticised of European nations not paying enough for their own defence and has threatened to withdraw support for Ukraine and even pull American out of the alliance.
