Ross Baillie, 21, a finalist in the 110-metre hurdles at last year's Commonwealth Games in Kuala Lumpur, suffered the reaction after eating a sandwich in Bath, where he was preparing for the European junior championships.
He was described as being in "critical condition" in the intensive care unit of the Royal United Hospital in Bath.
His parents, Hugh and Sheila, travelled from their home in Clydebank, near Glasgow, to be at their son's bedside.
The international swimmer Mark Foster was with Baillie for lunch when the hurdler fell ill.
Foster said: "We had finished training and went to get a sandwich. I chose coronation chicken and Ross had the same and we went back to the flat to eat them
"As soon as he bit into it, Ross said he realised there were nuts in it and his tongue started swelling and he started to cough.
"The coughing continued so I put him in the car and took him to the nearest doctors at the University in Bath as he was finding it increasingly difficult to breathe.
"He was quickly diagnosed and had an adrenaline injection, from there I helped carry him off to hospital at which point I realised it was not looking too good."
Jackson was expected to visit Baillie in hospital later today.
Baillie, a member of Glasgow's Victoria Park Athletic Club, broke the Scottish 110-metre hurdles record at the Commonwealth Games. He was tipped to win a gold medal at this summer's European junior championships in Finland.Reuse content