World War 1: UK remembers the day it entered the War

Lights go out as UK remembers WWI

Lights are extinguished across the country to mark the 100th anniversary of the start of the First World War, echoing the then-Foreign Minister Lord Grey eve-of-war observation that "the lamps are going out all over Europe"

Lights across Britain will be switched off for an hour on Monday in a tribute to the dead of World War One inspired by the prophetic observation of then-Foreign Minister Lord Grey on the eve of war 100 years ago.

"The lamps are going out all over Europe," Lord Grey said in 1914. "We shall not see them lit again in our lifetime."

Landmarks like Trafalgar Square will go dark from 10pm as people across the country switched off all but a single light in their homes for an hour.

The "war to end all wars" spread carnage across Europe, especially northern France and Belgium, killing 17 million soldiers and civilians in 1914-18. One million of the dead were soldiers from Britain and its then empire.

Mr Cameron and Prince William, second-in-line to the throne, attended 100th anniversary ceremonies in Scotland and Belgium on Monday. Speaking at an event in Liege, William paid tribute to those who died as he noted that the current fighting in Ukraine showed instability continued to stalk Europe.

"We were enemies more than once in the last century and today we are friends and allies," he said, alluding to Germany and its cohorts in the first and second world wars.

The theme of Grey's prophecy will also be at the centre of a service in Westminster Abbey on Monday as the building's lights will go out one by one until only a burning oil lamp remains at the Tomb of the Unknown Soldier.

At 10pm the lamp will be extinguished, marking the exact time the British Empire joined the war. In Trafalgar Square, one single light shone from an old police box.

The war's most enduring symbol, poppies, featured at the Tower of London with an art installation called Blood Swept Lands and Seas of Red by Paul Cummins in which ceramic poppies flow from the medieval monument's wall into the dry moat.

The artwork will grow throughout the summer until 888,246 poppies have been added to represent each British or Colonial fatality during the war – more than double the number of Britain's casualties in World War Two.

Red poppies have become a symbol of remembrance since the trench warfare waged in the poppy fields of Belgium's Flanders region during the 1914-1918 war.