Kate Middleton played an unofficial key role in her friend's wedding.

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Kate Middleton radiated supportive gal pal vibes at the wedding of a close friend on the weekend, with pictures emerging of her beaming as she watched the nuptials.

Attending the wedding of Sophie Carter and Robert Snuggs in Norfolk with Prince William, Prince George and Princess Charlotte in tow, she looked every bit the proud bestie we all deserve by our side.

But it appeared she was fulfilling her official best friend duties undercover.

Sophie is one of Princess Charlotte’s godmothers, and said to be one of Kate’s closest friends, but even though Prince George and Princess Charlotte made up the page boys and bridesmaids, Kate seemed to be absent from the bridal party.

Or was she?

As reported by the Mirror, people think Kate was actually unofficial matron-of-honour.

And the clue clever royal-watchers picked up was in her outfit.

Kate donned a blue Catherine Walker dress, matching coat and floral headband, which the amateur detectives among us think was chosen to match the bridal party.

The Duchess of Cambridge wore the same dress and coat pictured here, but with a matching floral headband. Source: Getty.

There was blue in the groomsmen's waistcoats, the page boys wore blue knickerbockers, and the bridesmaids (including Princess Charlotte, who looked freakin' adorable) had blue piping in their white dresses.

Kate's close friend Sophie Carter attended Prince Harry and Meghan Markle's wedding in May.

It wouldn't be the first time Kate has fulfilled the VIP matrimonial duty from the sideline - she wasn't matron-of-honour for her sister Pippa last year, either, but instead helped look after page boys and bridesmaids.

But why?

Well, back when Pippa tied the knot, it was reported that Kate didn't want to upstage her little sister on her big day.

It may be the same reason in Sophie's case, which we think is very considerate of Kate. (But we're also not surprised she's a top bestie and big sis).

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