The Prince of Wales has paid tribute to a girl known as "the 100-year-old teenager" as mourners gathered at her funeral.

In a letter to Hayley Okines's grieving parents, Charles described the 17-year-old who died this month after living a lifetime with progeria as "an inspiration to millions".

The condition, which affects about 125 children in 43 countries, causes people to age eight times faster than the usual rate.

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, where more than 250 colourfully-dressed mourners gathered.

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.

"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.

"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, including her mother, 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.