Jim Owers was helped to prepare for the seven-hour 50-mile trip by her, who rode as a passenger while he trained.

The youngster from Heathfield, Sussex, is facing her third major operation.

She is aware the gruelling ride by 27,000 cyclists over the Downs is for people like her. "He's cycling for people with special hearts," she said.

Her case has been featured on City Hospital, the BBC daytime programme broadcast from Guys and St Thomas' Hospitals in central London.

She is a joy to be with Jim Owers

Her father is riding with about 50 colleagues from work and hopes personally to raise more than £1,000 for the charity.

Since the race started in 1980, the event has raised more than £30m for the charity and has attracted more than 550,000 riders.

Mr Owers said: "On a day-to-day basis, she is quite breathless and the amount of exercise she can do is really quite limited.

"But in herself she is fantastic. She is like a normal three-year-old. She is a joy to be with."

Thousands of families, friends and supporters were expected to gather at the finish line at Marine Parade.

Water stops

Other vantage points for people who want to see the race are at Crawley Down and the nearby Turners Hill crossroads, and the top of Ditchling Beacon in the South Downs.

A race spokesman said the first of the riders started arriving to a "beautiful sunny day" in Brighton at around 1030 BST.

Cyclists were being urged to have a drink at the many water stops set up along the 54-mile route.