Sir Keir Starmer declared the UK was back playing a leading role on the world stage at the conclusion of his first international summit as Prime Minister.

Sir Keir said he would “reset” the UK’s relationship with Europe, “return to leadership” on climate change and “engage more deeply” with developing countries in the southern hemisphere.

His comments came at the conclusion of Nato’s 75th anniversary summit in Washington, where the Prime Minister said the allies had united behind Ukraine and the idea that a Russian victory is “unthinkable”.

The 32-strong alliance has put Ukraine on an “irreversible” path to full Nato membership and committed extra military aid.

Sir Keir said: “Together with our allies today, we have reaffirmed our unshakeable support for Ukraine’s ultimate victory.

“Our determination – to deliver justice for the awful crimes that Russia has committed.

“You will have all seen the scenes this week in Kyiv, Russia using some of the deadliest weapons in its arsenal on innocent children. Striking a hospital.

“The alternative to Ukraine’s victory is unthinkable.”

It would be an “affront to our values” and give “a green light to aggressors everywhere”, he warned.

Sir Keir has committed to provide £3 billion of military aid to Ukraine every year “for as long as it takes”.

At the Washington summit, the alliance as a whole promised a 40 billion dollar (£31 billion) annual support package, a new body to coordinate that help and measures to ramp up the industrial production of military equipment.

Sir Keir said the allies “confirmed Ukraine’s irreversible path to full Nato membership”.

The gathering in Washington was Sir Keir’s debut on the international stage, and his decision to highlight efforts to repair relations with Europe and put the fight against climate change at the heart of his diplomatic policy served to underline the break from his predecessor Rishi Sunak.

Sir Keir said: “Britain belongs on the world stage.

“So I am determined to reset our relationship with Europe, return to leadership on climate change and engage more deeply with the global south.”

He said he was “determined to renew Britain’s place on the world stage”.

He added that he was “proud of what we have to offer, confident, not just in the value of our strength, but in the strength of our values”.

The summit came “in a new and dangerous era” for the world, defined by “volatility and insecurity”, he said, adding that Vladimir Putin’s Russia poses a “generational threat”, supported by North Korea and Iran, while conflicts rage across the Middle East and Africa.

He also highlighted the “challenge” of China, terrorism and the undermining of international institutions.