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Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge leave at the end of a National Service of Thanksgiving for the 90th birthday of Britain's Queen Elizabeth II at St Paul's Cathedral in LondonSTEFAN ROUSSEAU/AFP/Getty Images

The Duchess of Cambridge was the main attraction at the National Service of Thanksgiving at St Paul's Cathedral in London marking the beginning of the Queen's 90th birthday celebrations. She was joined by other senior members of the royal family including her husband Prince William, Prince Harry, Prince Charles and the Duchess of Cornwall.

While the Queen smiled for the cameras alongside the Duke of Edinburgh in a resplendent embroidered yellow outfit, Kate Middleton opted for a custom-made powder blue coat dress by Catherine Walker adorned with an intricate white lace embellishment and V-neck design.

The Duchess oozed elegance as she accessorised the knee-length creation with a Jane Taylor ivory floral fascinator, tan suede court shoes and box clutch bag, wearing her long hair in a relaxed yet chic updo.

Middleton almost encountered a disastrous situation when her heel became wedged in a grate in the cathedral as she was accompanied by William and Harry, but was seen joking with her husband as she narrowly avoided the mishap.

Today's demure appearance came after her evening do at Kensington Palace last night (9 June) where she stunned guests in a £2000 royal blue floor-length gown by French fashion designer Roland Mouret. The frock showcased the Duchess' svelte physique and oozed contemporary elegance with its shoulder cut-outs and fluid silhouette.

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The Duke and Duchess of Cambridge at St Paul's CathedralSTEFAN ROUSSEAU/AFP/Getty Images

Middleton continued where she left off and became the star of the show as the ceremony, led by the Archbishop of Canterbury Justin Welby, got under way. During the service, he narrated how the Queen had ruled "through war and hardship, through turmoil and change during the course of her 90 years", before asking the congregation to reflect on her reign "with deep wonder and profound gratitude".

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Prince Harry,  Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge in coversation during the national service of thanksgiving for the Queen's 90th birthday at St Paul's CathedralStuart C. Wilson/Getty Images

The event was attended by a star-studded congregation comprising David Attenborough, Clare Balding, former prime ministers John Major and Tony Blair. Young royals such as sisters, Princesses Beatrice and Eugenie were also present, and looked sophisticated in their attires.

More faces from the political world made up the invited guests with senior figures of the Cabinet including Chancellor George Osborne and Labour leader Jeremy Corbyn.

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Prince Harry, Catherine, Duchess of Cambridge and Prince William leave a National Service of Thanksgiving as part of the 90th birthday celebrations for The Queen at St Paul's CathedralStuart C. Wilson/Getty Images

Archbishop Welby commenced the sermon by telling the the congregation: "Ninety years ago, Her Majesty The Queen was born, like every human being knit together in her mother's womb, and today we thank God for the way in which she, like every human being, is fearfully and wonderfully made.

"Before we ever come to light, God marks our journeys. No one at the Queen's birth knew for what she was destined. Today we recognise that God knew."

kate middleton
Duchess of Cambridge attends a National Service of Thanksgiving as part of the 90th birthday celebrations for the Queen at St Paul's CathedralStuart C. Wilson/Getty Images

He added: "We are here today to worship the God who made our Queen, and to celebrate the way in which God's hand has been so uniquely evident in her life."

Watch the highlights from today's live broadcast of the national service at St Paul's at 7pm on BBC2.

Kate Middleton
The Duchess of Cambridge attends SportsAid's 40th anniversary dinner at Kensington Palace on 9 JuneEddie Mulholland