Winchester schoolgirl Allegra Whittome, 16, died in her sleep last weekend days after being taken ill

Young athlete Allegra Whittome was a competitive 16-year-old who was renowned for her talents on the tracks.

But today her school and sports friends have been left devastated after hearing the shocking news that she had died suddenly - having fallen ill and pulled out of a competition hours before it was due to start.

Mystery surrounds exactly what caused the teenager's death which is now at the centre of an investigation.

Allegra, the daughter of a multi-millionaire former city banker, passed away last weekend just days after becoming unwell.

The Central Hampshire coroner in Winchester has been informed and inquiries are being made as to whether an inquest will be opened in coming days, said a spokesman.

Allegra was a year 11 pupil at St Swithun's private girls' school in Winchester.

She was a keen athlete competing on behalf of Winchester and District Athletics Club at the Bar End track.

In a statement, club chairman Pete Spelman said: “It is with great sadness that I have to inform you that one of our athletes, Allegra Whittome, passed away peacefully in her sleep yesterday.

“Allegra was a top all-round athlete that competed regularly for WADAC, Hampshire and St Swithun's School.

“Full of life and enthusiasm, Allegra loved competing in sport. She was a massive team player in David Vosser's and Pete Spelman's training squad and was loved by all her team mates.

“Only on Friday, Allegra's mum got in touch to say how sorry Allegra was not to be able to compete in the Hampshire League in Bournemouth on Saturday as she wasn't feeling too good. We shall all miss her big time.

“May Allegra rest in peace. Our thoughts are prayers are with her family at this very sad time.”

In 2013 Allegra raised money for the Little Princess Trust charity when she had 12 inches of her hair cut off by Guy Kremer.

Allegra lived in Lower Wield near Alresford and was the daughter of Lucy and Ashley Whittome, a former City banker with Credit Suisse First Boston and director of numerous Hampshire companies.

In 2011 a Hampshire lifestyle magazine estimated his personal wealth at £50 million.

She had been due to compete for her local athletics club at the weekend but had to pull out of the event after falling ill.

St Swithuns headmistress, Jane Gandee, said: "[Allegra was] an enormously talented and outstanding pupil both in the classroom and on the sports field, and her generous spirit touched everyone she met.

"She was so competitive while also open and kind to everyone, and with such determination to do the right thing - Allegra’s enthusiasm and determination were legendary.

"We all admired her steadfast moral compass, most recently demonstrated in the assembly she led on Tuesday morning.

"She had a wonderful sense of humour and she will be greatly missed. Our thoughts and prayers are with her family at this most difficult of times."

Allegra's family are said to be devastated by their sudden loss, but said in a statement: "Allegra was the most perfect daughter and sister that anyone could imagine. She was always smiling and full of enthusiasm for life.

"She loved sport, animals and her friends & family. Nothing will ever replace her.”