Kate Middleton stuns in Catherine Walker during the Queen's birthday celebration

Kate Middleton took on Queen Elizabeth's birthday celebrations in style! The Duchess of Cambridge was sheer elegance in Catherine Walker as she attended a service of thanksgiving at St. Paul's cathedral in London on Friday morning.

The 34-year-old royal fashionista looked stunning in the pale blue dress by one of her favorite designers, Catherine Walker. The ensemble featured white appliquéd lace detailing running the length of the dress, while a V neck added a modern touch to the formal number.

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Kate wore a pale blue Catherine Walker dress to the Queen's birthday celebration Photo: Getty Images

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Kate topped off the look with the white pillbox hat by her favorite milliner Jane Taylor which she also wore for Princess Charlotte's christening. Her famous locks were tied up into a chic chignon that ensured the stunning headgear took centre stage. Kate was accompanied by her husband Prince William as she arrived at the cathedral, and they joined a host of other royal family members to celebrate the Queen's 90th birthday.

Kate topped off the look with a pillbox hat by Jane Taylor Photo: Getty Images

Prince Charles, the Duchess of Cornwall and Prince Harry were all in attendance, while Prince Andrew and his daughters Princess Eugenie and Princess Beatrice also made up the royal party.

The service is held each year to mark the Queen's birthday and falls on the day before the traditional Trooping the Colour ceremony.

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The service celebrated the Queen's life with a series of prayers, hymns, readings and musical performances. It marks the beginning of a weekend of national and Commonwealth celebrations.

Kate attended the affair with Prince William and Prince Harry Photo: Getty Images

Prominent guests at the ceremony included Prime Minister David Cameron, who gave a reading from the New Testament, and the Archbishop of Canterbury, who preached the sermon.

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Kate was spotted last month wearing Catherine Walker when she recycled a green coat by the designer during the Chelsea Flower Show. The green dress made its debut during her 2014 royal tour of Australia and New Zealand during Will and Kate's visit to the Australian capital, Canberra.