Television sports presenter Charlie Webster is seriously ill in a Rio hospital after contracting an infection following a 3,000 mile charity cycle from London.
The 33-year-old became unwell on August 6 after watching the Olympic opening ceremony, having arrived in Rio a few days before.
She was admitted to hospital where doctors thought she was dehydrated following the gruelling six-week Ride to Rio challenge.
However her condition deteriorated after developing a severe complication from a bacterial infection.
Webster, a former Sky and ITV sports presenter, had cycled 1,600 miles from Recife in the north-east of the country to Rio de Janeiro 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 Rio as part of Team GB's Great Britons campaign alongside Nick Grimshaw and Jodie Kidd.
The marathon runner shared her frustration with her fans on social media the day after she was admitted to hospital, writing: "6 weeks on the road ends in this. Very rough day, severe dehydration & infection - 2 drips & antibiotics #Rio".
But Webster 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 Webster made no more posts and a statement on her Facebook and Twitter profiles on August 11 revealed how her condition had worsened.
It said: "Charlie has updated you all throughout her cycle and we know her silence was causing concern.
"On Saturday, 6th August, Charlie became unwell and was admitted to hospital in Rio de Janeiro.
"What was initially thought as dehydration, caused by her riding to Rio, has since been diagnosed as a severe complication caused by a bacterial infection.
"Charlie is receiving great care and we will update when we can."
Webster, who once dated Downton Abbey star Allen Leech, and Mike Tomlinson set off on the first leg of the Ride to Rio challenge on June 27, cycling 1,300 miles to Lisbon.
They cycled 90 miles a day with only one rest day each week before arriving at the Maracana stadium on August 4.
The Ride to Rio campaign wished Webster 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."