Prince Charles has led tributes to the girl dubbed the '100-year-old teenager' as hundreds of mourners gathered for her funeral. In a letter to Hayley Okines's grieving parents Charles described the 17-year-old progeria sufferer, who died earlier this month, as 'an inspiration to millions'. There are 74 known cases of progeria around the world and only 4 in the UK - it causes people to age eight times faster than the usual rate. Scroll down for video . Prince Charles' letter was read at Hayley's funeral by Father Michael Bailey at All Saints Church in Sidley, Bexhill-on-Sea, East Sussex. Her tiny turquoise coffin (pictured) is carried out of the church . In a letter (pictured) to Hayley's parents Charles described her as 'an inspiration to millions' In his letter the Prince said he had been 'deeply impressed by her incredible spirit and her infectious love of life' when they met at the Children of Courage event at Westminster Abbey in 2004. Charles' letter was read at Hayley's funeral by Father Michael Bailey at All Saints Church in Sidley, Bexhill-on-Sea, East Sussex, where more than 250 colourfully-dressed mourners gathered this afternoon. On Clarence House-headed paper, Charles told Hayley's parents Kerry and Mark: 'I know that she was an inspiration to millions in the way she dealt with the unimaginable challenge of progeria. 'You must be so very proud of the way that she used her brief life to make such a difference to the lives of others through her charity work and I do hope you can take some comfort, despite all the anguish you must be suffering, in her truly astounding achievements. More than 250 colourfully-dressed mourners gathered at the church for Hayley's funeral this afternoon . Hayley's mother Kerry watches as her daughter's coffin is carried into the church before the funeral . Hayley's mother Kerry (pictured) broke down as she paid tribute to her daughter at her funeral today . 'It is all the most tremendous credit to the unfailing love and care which I know your family gave her. I fear that under such unspeakably sad circumstances mere words are hopelessly and utterly inadequate. Prince Charles wrote a letter to Hayley's grieving parents calling her 'an inspiration to millions' 'So although I realise only too well that it will be of no consolation whatsoever, all we can do is to hold you in our most special thoughts and prayers at such a heart-rending time. 'Yours, with the greatest sympathy, Charles.' Hayley's tiny turquoise-coloured coffin was carried into church by pallbearers in front of mourners who had gathered from countries including America, Portugal and Belgium. Candles were lit before Hayley's younger sister Ruby sang a song called You're Free, which included the line: 'God takes the best ... there must've been a reason why they took you first.' In a personal address to the church, Ruby then spoke of her fond memories of Hayley, whose death prompted tributes from celebrities she had met, including Justin Bieber. Ruby told the congregation that 'she could not ask for a better sister', adding: 'Now we all have to be strong about it. I still can't get my head around it.' Hayley's mother broke down as she paid tribute to her daughter, telling mourners: 'This is the hardest thing I have ever had to do. 'I'm so proud to call you my daughter. You are the reason why I got out of bed in the morning and managed to put one foot in front of another.' Mrs Okines said she loved Hayley's 'sarcasm, dry sense of humour and stubbornness'. And she added: 'I can't imagine spending the rest of my life without you.' Mr Okines spoke of his 'very special girl' in a poem read out in church. He said: 'Hayley, if you can hear me, I love you so much.' A slideshow depicting some of Hayley's achievements and the people she had met throughout her short life were played on a big screen in church. The Prince of Wales mets Hayley at the annual Children of Courage Awards at Westminster Abbey in 2002 . When she was 13 Hayley met her favourite popstar Justin Bieber (both pictured) - much to her delight . Hayley and her parents (pictured) when she appeared in a documentary about her illness . They included cherished images of the youngster on holidays, on outings, with friends and family, and with celebrities including Kylie Minogue, Frank Lampard, John Terry and Norman Wisdom. Father Michael read excerpts from Hayley's book, Old Before My Time, published when she was 14, before paying tribute to her 'courage' in an address. He said: 'She has touched the hearts and minds of many people, in the parish, in Sidley, the nation and internationally, too. 'We all of us one day will depart this life and will we have the same courage as this young lady to face the problems of life so head on? 'Not to wish away our problems but to deal with them to directly with strong determination.' Father Michael said Hayley's 'memory and legacy' would never be forgotten. There are 74 known cases of progeria around the world and Hayley was one of only four in the UK . Floral tributes for Hayley pictured outside All Saints Church in Bexhill-on-Sea ahead of her funeral today . Hayley Okines' mother Kerruy leads friends and family as they arrive at All Saints Church in Bexhill-on-Sea . The hundreds of colourfully dressed mourners follow Hayley's coffin as it is taken away from the church . And he went on: 'Hayley's smile tells us that she lived an action-packed life. It is hard to keep up with all her adventures, as a 13-year-old girl she hugged Justin Bieber. 'She was a teenager with all the hopes and excitements of life. This is her memory, her legacy, to live for the day and to do so with courage.' Following prayers, Hayley's coffin was brought out of church to Don't Stop by 5 Seconds of Summer, before a private committal. Hayley and her family had fundraised for her medical treatment and to raise public understanding of the condition. Hayley became well-known three years ago when she was part of a documentary about her premature ageing condition. She suffered from the rare disease progeria which ages the body at eight times the normal rate. Progeria takes its name from the Greek word 'proeros', meaning prematurely old. It involves a mutant protein called progerin that accelerates physical ageing. There are around 74 known cases of the disease around the world and only four in the UK. 10 year old progeria child will have the appearance of an octogenarian with symptoms including baldness, arthritis and heart problems, but the mind of a 10 year old. Progeria patients normally die from heart attacks or strokes at an average age of 13. But Hayley, from Bexhill, East Sussex, defied the odds to live four more years and even published an autobiography about living with the disease. Her story was catapulted into the limelight after she starred in documentaries called The Girl who is Older than Her Mother and World's Oldest Teenager: Extraordinary People. Her autobiography, called Old Before my Time, detailed pop-star crushes and a dislike of school. But it was also a moving insight into how a child copes with a disease which resulted in Hayley having the body of a 105-year-old. In an interview with The Sunday People in 2013, her mother described the hell of living with the disease. 'The most difficult part is Hayley has the mind of a normal 13-year-old girl locked in an old person's body,' she said. 'Someone else has to get her a drink as she's too small to reach taps by herself. 'And buying fashionable clothes is tough because she still wears clothes for a five-year-old. Hayley, who turned 17 on December 3, left school last summer. However, despite pioneering drug treatment in the US that gave her a new lease of life, she was unable to conquer the disease. Her health deteriorated and she contracted pneumonia, and died at home on April 2. Her death was announced on April 3 when her mother Kerry posted on Facebook: 'My baby girl has gone somewhere better. She took her last breath in my arms at 9.39pm x.' Hayley's parents have previously described the heartache her condition had caused. Her mother said: '[When she was born] she had fine blonde hair, blue eyes and her father and I were besotted with her. 'At 10 months she walked for the first time – we were so proud. 'The only concern was she was so petite and didn't appear to be growing. 'But as I'm only 5ft 4ins I tried to tell myself she simply took after me.' Hayley and her family fundraised for her medical treatment and to raise public understanding of the condition . At 13 months, Hayley still wore clothes for a three-month-old, so her mother took her to their GP. Tests revealed she had a rare gene mutation called Hutchinson-Gilford progeria. Next came the devastating news that Hayley would only live until the age of 13. Determined to make the most of the time she had, the family embarked on giving Hayley amazing memories. Members of the local community raised thousands of pounds to pay for her to go to Disneyland in California. In her short life, Hayley went swimming with dolphins, travelled all over the world, met Prince Charles, Kylie Minogue and Justin Bieber and starred in several TV documentaries. Despite her terrible prognosis, the teenager remained upbeat. 'In many ways I feel I've been lucky. Because of this I've met more people and done more than many do in a lifetime,' she said.
Girl dubbed '100-year-old teenager' Hayley Okines died earlier this month . She suffered from progeria which ages body eight times the normal rate . Prince Charles sent letter to her parents which was read at her funeral . In it he called the brave teenager 'an inspiration to millions'