Sir Keir Starmer has accused Donald Trump of badmouthing him in public in an attempt to drag Britain into his war against Iran.
The US president has repeatedly dragged the PM in recent days over the UK's refusal, along with other NATO members, to help out with the conflict he launched in the Middle East.
But Sir Keir used an interview to vow he would not 'buckle' and repeated his vow that while US forces will be allowed to use UK bases our own Armed Forces will not be dragged into a 'wider' conflict.
In his latest attack last night Trump branded the UK's two largest warships, the aircraft carriers HMS Queen Elizabeth and HMS Prince of Wales, 'toys'.
He has also repeatedly decried Sir Keir as 'no Churchill' who made 'a big mistake' in not rushing to support the US-Israeli onslaught.
The rift has left the transatlantic 'special relationship' at its lowest ebb in decades, with the US increasingly isolated over a conflict that is threatening to plunge the global economy into recession.
Asked whether Mr Trump's repeated attacks had made him bristle, the Prime Minister told Sky News' Electoral Dysfunction podcast: 'I think I understand what's happening, it's to put pressure on me in different ways.
'But, that pressure isn't going to make me waver. It's not going to make me abandon my principles or values, and that's just the way I am.
Sir Keir used an interview to vow he would not 'buckle' and repeated his vow that the UK will not be dragged into a 'wider' conflict
The US president has repeatedly dragged the PM in recent days over the UK's refusal, along with other NATO members, to help out with the conflict he launched in the Middle East
'That is not new. That isn't because of President Trump. I've got core values and principles I've held all my life, and they're irreducible.'
He acknowledged there was 'a clear difference of opinion' between himself and the president and that there had been 'hard decisions' over recent weeks, but insisted he still wanted a 'good relationship'.
'My own view is that a lot of what is said and done has been to put pressure on me to change my mind, but I'm not going to do so, because I'm the British Prime Minister and I have to act in the British national interest, and I will always act in the British national interest,' Sir Keir said.
Speaking at a televised White House Cabinet meeting yesterday, Trump lashed out again at Nato for doing 'absolutely nothing' to help the US in the Middle East conflict.
And he took aim at the UK's aircraft carriers, HMS Queen Elizabeth and HMS Prince of Wales.
He said: 'Now they all want to help. When they're annihilated, the other side is annihilated, they said, ''we'd love to send ships''…
'We had the UK say that ''we'll send'' – this is three weeks ago – ''we'll send our aircraft carriers'', which aren't the best aircraft carriers, by the way. They're toys compared to what we have.'
Meanwhile, US defence secretary Pete Hegseth suggested that Iran has the capability to strike London and insisted the president 'knows better' than others about the threat posed by Tehran.
Yesterday, his British counterpart, John Healey, refused to say if Iran was able to hit the UK, but said military chiefs did not believe the country had any plans to do so.
The destroyer HMS Dragon (pictured in March) was due to lead a Nato mission in the North Atlantic before it was redeployed to Cyprus earlier this month in the wake of the Iran conflict
Keir Starmer has been accused of overseeing a defence 'shambles' over the state of the Royal Navy after it emerged Britain had to ask Germany for the use of a warship to keep a major Nato commitment.
Germany has deployed the frigate Sachsen with British officers aboard as the flagship of an alliance maritime task group in place of HMS Dragon, which was sent to help defend Cyprus from Iranian attacks.
But the move billed by Berlin as 'an expression of the close (Germany-UK) partnership' has raised alarm bells about how many warships the UK can field at a time.
Mr Healey was unable to say how many of the Royal Navy's surface fleet of 17 frigates and destroyers is currently fit to fight, when grilled over the state of the service.
It came as Sir Keir this morning again refused to say when he will reveal the much-delayed blueprint for increasing spending on the UK's Armed Forces, even as he warned of the threat posed by Russia to world peace.
Labour's defence investment plan (DIP), showing how it plans to push military pending to 3.5 per cent of GDP, was due to be published last autumn but has since been repeatedly pushed back.
Additionally, new figures released by NATO today showed that UK military spending as a proportion of GDP was lower than expected last year, at 2.3 per cent rather than the expected 2.4 per cent.
