TV presenter Charlie Webster has fallen into a coma in a Rio de Janeiro hospital after contracting a rare strain of malaria.
The 33-year-old Team GB ambassador is on life support after becoming unwell while watching the opening ceremony of the Olympic Games.
Her mother, brother and manager have flown from the UK to be by her bedside. Advice has also been provided by the London School of Tropical Medicine.
The former Sky and ITV sports presenter fell seriously ill on August 6 following a 3,000-mile (4,828km) charity cycle ride, having arrived in Rio a few days earlier.
Charlie was admitted to hospital, where doctors initially thought she was dehydrated following the gruelling six-week Ride To Rio challenge.
However, her condition deteriorated after she developed a severe complication from a bacterial infection.
On Thursday, a message posted on her Twitter page read: "Thank you very much for all your messages of support, it means so much. #TeamCharlie."
Charlie had cycled 1,600 miles (2,575km) from Recife in the north-east of the country to Rio in 39 days with the widower of breast cancer fundraiser Jane Tomlinson.
She appeared to be in good health when she arrived in the Brazilian capital on August 4 and was due to stay on in the city as part of Team GB's Great Britons campaign alongside DJ Nick Grimshaw and model Jodie Kidd.
The marathon runner shared her frustration with her fans on social media the day after she was admitted to hospital.
But Charlie remained positive, telling fans on August 9: "I'm getting there ... awful few days with serious infection #Rio."
She posted a photo showing her lying in a hospital bed hooked up to a drip, but smiling, adding later: "Looking forward to being a part of @TeamGB here at #Rio2016."
However she made no further posts and a statement on her Facebook and Twitter profiles on August 11 revealed that her condition had worsened.
Charlie, who once dated Downton Abbey actor Allen Leech, set off on the first leg of the Ride To Rio challenge on June 27, cycling 1,300 miles (2,092km) from London to Lisbon.
They cycled 90 miles (145km) a day with only one rest day each week before arriving at the Maracana stadium on August 4.
The Ride To Rio campaign wished Charlie a speedy recovery via Twitter, describing her as a "tough lady".
A Foreign Office spokesman said: "We are providing support to a British woman who is in hospital in Rio."