Sacha Dench, who was attempting a world-first circumnavigation of mainland Britain to raise awareness about climate change, has been seriously injured and a member of her support staff, Dan Burton, has been killed after a paramotor accident during their challenge.
Ms Dench had been dubbed the "human swan" as she attempted a 3,000-mile journey to mark the COP26 UN climate change conference on 31 October.
In a statement, the trustees of Conservation Without Borders said the accident happened in the western Highlands of Scotland near Loch Na Gainmhich.
The statement said: "We are very sorry to have to confirm that Dan Burton, the support paramotorist, has died as a result of the accident.
"Sacha Dench is seriously injured and is being treated in hospital. Her injuries are serious but not life threatening.
"Both highly experienced paramotorists, our thoughts are with the family of Dan Burton, to whom we offer our sincere condolences.
"The incident was attended by police and medics and inquiries are under way to establish the details of the accident."
The statement added that the families of those involved had been informed and that the Round Britain Climate Challenge would now be put on hold.