Sir Keir Starmer insisted Nato remains committed to collective defence after US President Donald Trump cast doubt on his support for the mutual aid pact.
Before Wednesday鈥檚 Nato summit in The Hague, Mr Trump had told reporters on the flight from Washington it would depend 鈥渙n your definition鈥 of Article 5, when asked if he would commit to collective defence.
Nato鈥檚 Article 5 commits all members of the alliance to come to the defence of each other if attacked.
But speaking to broadcasters on the way into the summit, the Prime Minister said allies were 鈥渁bsolutely committed to the importance of Nato, particularly in a very volatile world鈥.
And before the meeting of the North Atlantic Council on Wednesday, Sir Keir appeared to be in deep conversation with Mr Trump.
Sir Keir鈥檚 message was repeated by Nato secretary-general Mark Rutte, who opened the summit meeting with the words: 鈥淥ur joint commitment to Article 5, that an attack on one is an attack on all, sends a powerful message.鈥
Standing next to Mr Trump, the secretary-general praised the US president for bringing about a commitment from the alliance to spend 5% of GDP on defence, telling him: 鈥淵ou made this possible.鈥
Earlier, Mr Rutte had described the US president as 鈥渁 man of strength鈥 as well as a 鈥渕an of peace鈥, praising him for securing a ceasefire between Israel and Iran.
Mr Trump also compared Israel and Iran to 鈥渢wo kids in a school yard鈥 fighting.
鈥淭hey fight like hell. You can鈥檛 stop them. Let them fight for about two, three minutes. Then it鈥檚 easier to stop them,鈥 he said.
Mr Rutte added: 鈥淭hen daddy has to sometimes use strong language.鈥
The UK has signed up to the new Nato spending target, pledging to spend 3.5% of GDP on 鈥渃ore defence鈥 and another 1.5% on security and resilience measures such as critical national infrastructure, border security and intelligence.
But the Government does not expect to reach the target until 2035, three years after the 2032 deadline some allies were pushing for, and has not said how the increase will be funded.
Alongside the commitment to increase defence spending, the UK has also announced its intention to purchase 12 new fighter jets capable of carrying nuclear weapons.
Billed as 鈥渢he biggest strengthening鈥 of Britain鈥檚 nuclear posture 鈥渋n a generation鈥, the move will see the jets committed to a Nato mission that would see US nuclear bombs fitted to allied aircraft.
It follows a recommendation from the strategic defence review, published earlier in June, to begin negotiations with the US and Nato on joining the alliance鈥檚 nuclear mission.
Following the announcement, Defence Secretary John Healey was seen speaking to his US counterpart Pete Hegseth on the margins of the Nato summit.