The 30-year-old fitness fanatic has been diagnosed with an irregular heartbeat but she refused to let it stop her running, a friend has said.
Tests are still being carried out after she collapsed less than a mile from the finishing line in the 26-mile marathon on April 22.
“Claire had been diagnosed with an irregular heartbeat,” a friend told
"It might have made a lesser person take a back seat. But Claire just got on with it."
Simon Van Herrewege said he was waiting to cheer Miss Squires over the finishing line when a nurse phoned to say she was “very poorly” and had been rushed to hospital.
“I thought she had twisted an ankle or something, and I ran and flagged down a taxi,” Van Herrewege said.
But when he arrived at the accident and emergency unit, he was informed that his girlfriend of nearly three years had died.
Miss Squires was running the London Marathon for the second time when she fell to the ground on Birdcage Walk, near Buckingham Palace.
She had been treated by medical teams situated along the route before she was rushed to hospital, where she was later pronounced dead.
Squires was running to raise funds for the Samaritans in honour of her brother Grant, who died of a drug overdose in 2001, aged 25.
Before the race she had raised £500 but, following news of her death, thousands of well-wishers have left donations totalling more than £1 million.
Mr Van Herrewege added: “"She'll be up there in heaven whooping for joy at the amazing worldwide response to her charity appeal. She is still giving."
Her death was the tenth during the London Marathon since it began in 1981.
The last competitor to die was a 22-year-old fitness instructor in 2007.