US President Donald Trump has previously questioned whether European partners would adhere to the commitment to defend its allies if called upon, known as Article 5.
The alliance was formed in 1949 with the primary aim of blocking expansion in Europe by the former Soviet Union.
Numbering 32 members, including several Eastern European countries, Article 5 is one of Nato’s core principles, external. It says that an armed attack against one or more members will be considered an attack against all.
But Starmer sought to dispel Trump’s doubts, telling the conference that the UK’s commitment to Article 5 is “as profound now as ever”.
He added: “Be in no doubt, if called on, the UK would come to your aid today.”
Delivering her speech prior to the PM, EU Commission President Ursula von der Leyen described Starmer as an “unflinching ally and friend,” and said both the EU and UK faced the threat of “outside forces trying to weaken our union from within”.
She said Europe had undergone “shock therapy” from the US and must take more responsibility for its defence.
“Europe needs to step up and has to take on its responsibility,” she added.
Von der Leyen also called for more pressure on Russian President Vladimir Putin, posting on X: “Now is the time to raise the costs of Russia’s war higher than ever.”
Downing Street said the pair met in Munich on Saturday and “agreed that Europe needs to step up and work towards becoming a more European Nato”.