Last month, French President Emmanuel Macron described Nato as “brain dead”, saying alliance members were no longer co-operating on a range of key issues.
US President Donald Trump, who has repeatedly clashed with Nato’s European members over the funding of the bloc, is among world leaders attending.
He arrived in the UK on Monday ahead of a reception to be hosted by the Queen at Buckingham Palace on Tuesday evening. Mr Trump is set to hold separate talks with the German and French leaders, and Nato’s secretary-general.
Nato members pledge to come to the aid of one another should any ally come under attack.
What will Boris Johnson say?As host of the two-day summit, Mr Johnson is expected to reiterate the importance of Nato staying united.