Joined by Prince Harry, the royals walked, heads bowed slightly, among a sea of ceramic poppies which poured out of the Tower into the moat below to symbolise the hundreds of thousands of lives lost in the conflict.
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Kate, who had recycled a deep blue LK Bennett dress for the occasion, was seen chatting with the Beefeater guards who are on duty at the Tower.
Entitled Blood Swept Lands And Seas of Red, the exhibition will eventually be made up of 888,246 flowers representing the British and Commonwealth service personnel who died during the war. There are currently 120,000 in place.
Each one was handmade in Derby by volunteers with a family link to the heroes who paid the ultimate sacrifice. This way no two are the same.
William was heard telling creator Paul Cummins that the piece was "spectacular" before joining him to climb the Middle Tower to view the artwork from on high.
More flowers will be planted throughout the summer before the last one on Armistice Day on November 11, marking the official end of the conflict. The poppies are available to buy for £25 as part of a fundraising drive for Help for Heroes and other Armed Forces charities, which the Princes support.
On Monday, the royal trio were in Belgium, where they took part in a moving twilight ceremony, lit only by lanterns to remember the dead.
More than 50 heads of state, including hosts King Philippe and Queen Mathilde looked on as the Duke of Cambridge made a speech, praising the European country for its continued remembrance of World War I.
The Duke pointed out that "in Ypres, Belgian volunteers have played the Last Post every night since 1928 – except during the Second World War".
He went on to quote British nurse Edith Cavell, who "saved soldiers from each side. On the night before she faced a German firing squad, she said: 'I must have no hatred or bitterness towards anyone'".