In black and white pictures shared by photographer and family friend Misan Harriman, Lilibet could be seen wearing a cardigan and what appeared to be bloomers under a dress.
WATCH: Harry and Meghan share photo of Lilibet on first birthday at Frogmore Cottage
Now, Instagram page @foundbybojana has revealed that the young girl was wearing Petit Bateau's Babies' Organic Cotton Knitted Cardigan in Herbier Green, paired with their cotton gauze bloomers.
Founded by Guillaume Darrousez, the brand has a store in Beverly Hills and several in London, including one in Chelsea near to Buckingham Palace.
The brand is also focused on sustainability, sharing on ther website that "more than ever before, we want to invest in the circular economy".
"We're optimistically COMMITTED to a model that helps protect the environment and future generations alike," they shared. "I believe these are the best ways to keep moving forward to enable children to have carefree fun in the natural surroundings we must protect. Liberty. Quality. Durability."
Lili wore the outfit for her first party (Photography: Misan Harriman)
Lili wore the outfit for the party at their UK home, Frogmore Cottage.
The backyard picnic was attended by close friends and family, and while not many details have been revealed, we do know that kids were treated to face painting.
Petit Bateau's Babies' Organic Cotton Knitted Cardigan (£39/$48) and Petit Bateau's cotton gauze bloomers (£19/$28)
The family also released a gorgeous photo of their daughter, taken by family friend Misan, who attended the casual, intimate backyard picnic alongside his family.
In the snap, the young girl wore a sweet baby blue dress crafted by Californian-based designer Isabel Garreton, with a white bow in her hairindependent brand Village Baby – which has been selling out rapidly thanks to Duchess Meghan’s impeccable taste.
She has gorgeous ginger hair, similar to her father Harry and older brother Archie.
Harry and Meghan also released the first portrait of the young girl (Photography: Misan Harriman)
It is unclear if Lili's uncle and aunt, Prince William and Kate Middleton, or her cousins Prince George, Princess Charlotte and Prince Louis met her during the celebrations, as the family were taking part in the four-day Platinum Jubilee events for the Queen.
She did reportedly meet her great-grandmother the Queen during the weekend, and speaking about the happy first meeting, HELLO!'s Royal Editor, Emily Nash said: "This will be a highlight of the jubilee celebrations for the Queen - her great-grandchildren mean the world to her and like any great grandmother who hasn't been able to meet the youngest member of the family yet, the moment when it finally happens will be full of joy."
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