Nato leaders are to meet outside of London for formal talks on Wednesday, as they gather to mark the military alliance's 70th anniversary.
The talks will take place at a luxury hotel near Watford and follows receptions held Tuesday at Buckingham Palace and Downing Street.
The two-day summit has so far been overshadowed by tension between allies.
Tuesday saw sharp exchanges between US President Donald Trump and French President Emmanuel Macron.
The highly choreographed anniversary gathering, intended to show unity, has been unable to hide deep differences between Nato members, BBC Defence Correspondent Jonathan Beale reports.
Although the 29-member bloc's future is not in doubt, there are disagreements over Turkey's recent military action in northern Syria, relative levels of military spending by members and recent controversial comments by President Macron that the alliance is “brain dead”.
Leaders will meet for a “family photograph” and a three-hour formal meeting which is expected to cover issues such as cyber-attacks and the strategic challenge posed by China.
UK Prime Minister Boris Johnson, the host, is expected remind Nato leaders that their commitment to defend one another is at the heart of the alliance and protects “nearly a billion people” under a principle of “one for all, and all for one”.
