Mr Chamberlain described his declaration of war as a "bitter blow"
The moment Britain finally declared war on Nazi Germany exactly 70 years ago is being remembered.
Prime Minister Neville Chamberlain made his sombre radio broadcast at 11.15am on 3 September 1939, two days after German forces attacked Poland.
France followed suit hours after the historic address at 10 Downing Street.
There are no official events to mark the anniversary, but war-related re-unions and debates are being held in northern England and London.
A group of land girls who worked together on farms in Lincolnshire during the war will be reunited in Grimsby - some for the first time in 70 years.
The former Land Army girls, who ploughed fields, dug up potatoes, tended animals and harvested crops, will enjoy a £2 wartime lunch at a care home.
Historians will discuss the importance of Britain's wartime leader, Winston Churchill, in a debate at London's Central Hall in Westminster.
And the Royal British Legion's 14th annual bike ride from Greenwich in London to Paris - to raise cash for past and present service personnel - is, by coincidence, also being held.
More than 170 cyclists, who hope to raise £300,000, will leave Dover for Calais on Thursday and are expected to arrive in Paris on Sunday lunchtime.
On Tuesday Foreign Secretary David Miliband joined world leaders - including German chancellor Angela Merkel - at Gdansk in Poland to mark the 70th anniversary of the start of the war.
In his historic broadcast from the Cabinet room, Mr Chamberlain admitted his "long struggle to win peace" had failed.
He said: "This morning the British Ambassador in Berlin handed the German government a final note stating that, unless we heard from them by 11 o'clock that they were prepared at once to withdraw their troops from Poland, a state of war would exist between us.
"I have to tell you now that no such undertaking has been received, and that consequently this country is at war with Germany.
"You can imagine what a bitter blow it is to me that all my long struggle to win peace has failed.
"Yet I cannot believe that there is anything more or anything different that I could have done, and that would have been more successful."
On Friday the Citizens Advice Bureau (CAB) will also mark its 70th anniversary - the day after the famous declaration.
The organisation started out as an emergency wartime service, launching 200 offices on 4 September 1939 as a result of government war planning.
It was run by the voluntary sector to help civilians trace missing relatives and cope with the huge disruption, evacuations and bombings.
The Second World War lasted nearly six years and cost around 50 million lives with an estimated 400,000 military and civilian casualties from Britain.