Neville Chamberlain's 1939 diary, in which the then prime minister marked the start of World War II by writing "War declared", is to go on display.
The diary will be part of an exhibition at London's Imperial War Museum.
The exhibition, Outbreak 1939, is being held to mark the 70th anniversary of the start of the war on 3 September.
The jacket worn by King George VI in his broadcast to the nation and a medal awarded to the first British soldier to be killed in action will be exhibited.
The exhibition also looks at the build-up to the war.
Among the items exhibited will be the then-Foreign Secretary Lord Halifax's letter to Germany formally declaring war.
In addition to the pencilled diary entry, visitors will be able to see a letter Mr Chamberlain wrote to his sister a week after the war was declared, describing life as "one long nightmare".
The MP for Edgbaston wrote: "Of course the difficulty is with Hitler himself. Until he disappears and his system collapses there can be no peace."
The museum's historian, Terry Chapman, said: "He did unburden himself to them and described the events of the week and what a dreadful thing after fighting so hard for peace ever since becoming prime minister in 1937.
"But especially at Munich in 1938 he is forced to become a war leader that he knows really he is not cut out to be."
A documentary featuring Tony Benn, Vera Lynn and Nicholas Parsons recounting their memories of the start of the war will also be shown.
The exhibition opens to the public on 20 August.