The Queen paid tribute to the "resilience and determination" of 9/11 survivors as the Prime Minister denounced the terrorists who killed nearly 3,000 people during the attacks.
During the changing of the guard at Windsor Castle, the Old and New Guard played the US National Anthem at 11am - the same as it was at Buckingham Palace on 12 September 2001.
The Prime Minister recorded a video message that played at a memorial event at the Olympic Park in east London where a memorial forged from the steel from Ground Zero - where the World Trade Center stood in New York - is on display.
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