The Countess of Wessex appeared to share a personal tweet as she celebrated her 57th birthday on Thursday.
The royal family reshared a special video message to Sophie from children supported by one of her patronages, OSCAR International.
"Thank you everyone for the lovely birthday wishes today!" the tweet read, suggesting it had been penned by the Countess herself.
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The Queen's daughter-in-law spent the day volunteering at a St John Ambulance vaccination centre, where she greeted patients, took their details and looked after their wellbeing.
Ann Cable MBE DL, Chief Commissioner at St John, said: "We are honoured that The Countess of Wessex chose to spend her birthday with St John. It is a testament to the outstanding contribution of our volunteers."
Sophie thanked the public for their birthday wishes
At the end of the shift, the royal mum-of-two was presented with a pin badge, which is given to all St John Ambulance personnel to thank them for their support with the vaccination programme, as well as a bouquet of flowers to mark her birthday.
The organisation also shared a video message from Sophie, where she thanked volunteers for their efforts and urged the public to get involved with volunteering.
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Sophie volunteered at a St John Ambulance vaccine centre
The Queen also posted a touching message and photograph on the royal family's social media channels to celebrate Sophie's birthday, writing: "Wishing The Countess of Wessex a very Happy Birthday today!"
The accompanying photograph showed Sophie cuddling a puppy during a visit to the Guide Dogs National Centre in Leamington Spa last year.
Sophie married the monarch's youngest son, Prince Edward, in Windsor in June 1999 and the couple share two children, Lady Louise Windsor, 18, and James, Viscount Severn, 14. The family-of-four reside at Bagshot Park in Surrey.
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