Johnson, a vocal supporter of Ukraine, accused NATO of ignoring the lessons of history and failing to send a strong message to Putin at the NATO summit in Vilnius.

Ukraine had hoped to receive a clear declaration from NATO members that it could become a member once its war with Russia ends, but the communiqué released by allies fell short of this, promising only that NATO would "be in a position to extend an invitation to Ukraine when allies agree and conditions are met." Frustration over the process was expressed by Ukrainian President Volodymyr Zelenskyy, who said there appeared to be "no readiness" to welcome the country into the alliance despite its battle with Russia.

Johnson argued that no country has "done more or tried harder than Ukraine to demonstrate its fitness for NATO membership" than the UK, and called on NATO to set out a timetable for membership as soon as the war is won.

Johnson's successor-but-one, Rishi Sunak, has defended the summit as a success, but critics have pointed to NATO's failure to offer a clear roadmap for Ukraine's admission as a setback for the country's hopes of joining the alliance.