It was 100 years ago, on April 1 1918, that the Royal Flying Corps and the Royal Naval Air Service merged to create the RAF - the world's first independent air service.
The Archbishop of Canterbury, Justin Welby, addressed the congregation and said the RAF has in its first 100 years been a decisive factor in serving the country, as well as being key in protecting democracy and freedom.
She wore one to Princess Charlotte's christening in July 2015 and repeated the look for the Trooping of the Color in July 2016 and a commemoration ceremony in Belgium in 2017.
Prince Harry and his brother Prince William both wore full RAF uniforms to the event, while Duchess Kate wore an Alexander McQueen number for the second day in a row, according to People. In addition, the Queen has entrusted Kate's prestigious diamond brooch, which was decorated with the attire of the Duchess.
After the service, the monarch, accompanied by Prince Charles, will present a new Queen's Colour to the RAF on the forecourt of Buckingham Palace and make a short speech.
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The Duchess of Cambridge made a surprise appearance at the centenary celebrations for the Royal Air Force yesterday.
The family has a long-standing commitment to the Royal Air Force with four generations of the family having served or trained with the United Kingdom's aerial warfare force, and many holding honorary ranks and patronages with RAF charities and associations.
The Duchess of Cambridge paused her maternity leave on Tuesday for a special occasion, close to her heart - literally.
She was spotted on a balcony with other Royals watching the flypast.