She has become known for her sense of thrift after shopping on the High Street and recycling many of her designer outfits.
But now the Duchess of Cambridge has proved her green credentials by wearing ethical fur, taken from alpacas, who die from natural causes - rather than being bred for their pelts.
MailOnline has discovered that the silver grey alpaca fur hat, which Kate teamed with her recycled £1,541 (NZ$2,671) moss green Sportmax coat, when she joined the royals and Middleton family at church in Sandringham, was made in Sicuani, in the depths of Peru under the Fairtrade label by local artisans using alpacas, killed in the harsh climate.
Kate, who is 35 today, bought the £320 (NZ$554) hat from milliners Lock & Co, which has Royal warrants from the Duke of Edinburgh and the Prince of Wales - she has more than a dozen hats from the company, including her favourite black Fairytale headpiece and brown Betty Boop hat.
But, in a twist, the Sumac Alpaca Fur Hat was not designed by royal favourite Sylvia Fletcher but by a Peruvian designer, Antonia Valentin Jacob, who grew up in an impoverished and violent shanty home in the capital Lima.
Antonia visits the alpacas in the freezing mountains of Sicuani, 11,000ft above sea level three times and year and insists she only uses animals that have perished in the harsh climate.
Unlike llamas, which are larger than alpacas and bred to be domesticated "beasts of burden", alpacas are bred purely for their fleeces.
Farmers sheer them annually in the spring and sell their shearlings to manufacturers, who make blankets, sweaters, ponchos, gloves, hats and scarves out of them.
The animals, which can live for up to 17 years, run free and graze on the level heights of the Andes, in southern Peru - they are not penned in so they can descend to the plains to nibble.
However they have to cope with extreme temperatures, which drop during the winter months to minus 20 degrees centigrade.
Natural selection means that baby alpacas struggle to cope with the freeze - some are not hardy enough to survive and die from cold, mudslides or flooding; others from starvation as they struggle to forage for food in the icy conditions.
Now Antonia, 52, who runs the company Lacorine - a twist on the name of her teenage daughter Corinne - has been innundated with orders for the hats, which come in all colours.
Last night she said: "It is the perfect start to 2017. We have really been affected by Brexit because of the devaluation of the pound and I was starting to think: 'What on earth am I going to do? I went to bed late last night because I was on the phone to the artisans in Peru. But I was woken up in the middle of the night by my phone going ping ping. I was getting lots of orders for the grey hat."
The Lacorine hat was first discovered by the Middleton sisters five years ago when they were shopping at William Evans & Co, which has a shop in St James'.
Kate bought the Sumac hat in black, chocolate brown and champagne, while Pippa wore a brown one to the Cheltenham races in 2013.
"They are fantastic hats," added Antonia, who lives and works with her husband Simon, a former property developer in West Wickham, Kent. 'They are made by people who can't read or write.
"They are artisans who have been in the industry for years and their skills have been passed down from generation to generation.
"The alpacas are not bred for slaughter. They die of natural causes in the harsh weather conditions and the farmers suffer terrible losses because they make money from sheering them for their fleeces. I visit them three times a year, flying to Cusco and take a bus to Sicuani to work with them. The ideas come from two worlds."
It has been a long journey for Antonia from her childhood in Lima to life in Kent. "I had no real life with my parents," she said.
"My mother can't read or write and my father was extremely violent. He was a monster. I think he was schizophrenic. We were five kids living in Lima in a shanty town house.
"My life changed at the age of 15 when I persuaded my father that I should get a job to help out with the bills. I met Renée, who came from an aristocratic family, and she took me on as their nanny to look after her grand-daughter Maria Trilse.
"For the first time I had my own bed and bedroom. I would spend the day at her house and the evenings at her daughter Vivianna's. She always treated me like family and told me I could live with her as long as I wanted."
She encouraged Antonia to go to school and she ended up studying economics at Lima University, where Vivianna's husband Ludovico was a lecturer.
When he came to Britain, to study for his doctorate at Brighton University, they employed her as an au pair. She arrived in Britain on January 3, 1987, with a temperature of 102, just before the birth of their second child Ludovico, and has never left.
"I didn't speak a word of English," she said. "But I learnt by watching the BBC and bought a book to study the grammar."
She met her husband, a former pupil at Alleyn's public school, at the Bahia salsa club, in Vauxhall.
The couple got married at Bromley register office in 1993 and had their reception at London's Inner Temple, where her father-in-law Isaac was a barrister.
However, in a twist of fate, Antonia now supports her benefactor Renée, now 95, after she and her husband Enrique fell victim to American fraudster Bernie Madoff, who was jailed in 2009 after admitting turning his wealth management business into a Ponzi scheme.
"It is terribly sad," she said. "They were incredibly well off and aristocratic. Then they met Bernie Madoff and invested in a chemical factory. They put all their money into it and lost everything.
"They had to sell their home and couldn't afford private nursing care. Enrique passed away last year and I send Renee money to help with the bills. I am going to send her a photograph of Kate wearing my hat. She will be thrilled."