Team GB decathlete Ben Gregory is in a coma and on life support after fracturing his skull in a horrific bike crash.
The injuries - including multiple brain haemorrhages - came after he was knocked over by a car.
His fiancee Naomi Heffernan said she had ‘never been so scared’ and he needed ‘positive thoughts and prayers’.
She wrote: “Ben was in a serious bike accident. He has a fractured skull, neck and multiple haemorrhages to his brain. He’s in a coma and on life support.”
She added: "Ben needs your positive thoughts and prayers right now. Those that know Ben know how strong he is mentally and physically, he's a fighter and he'll get through this.
"Please pray for him and send all the positive energy and thoughts you can muster." Welsh Athletics paid tribute to Gregory in a statement, saying: "Our thoughts and prayers are with Ben's friends and family at this time.
"Ben's accomplishments as an athlete speak for themselves: Wales' greatest ever decathlete - three Commonwealth Games, the Welsh record holder and representing Wales and Great Britain multiple times. The outpouring of support across social media is testament to his popularity as a person and as an athlete.
"Everyone who has had the privilege of training or competing alongside Ben will speak of his incredible love of life, his kindness, support and loyalty. We will all continue to pray for a full recovery for Ben."
Former Team GB heptathlete Kelly Sotherton said Gregory had been hit by a car, tweeting: "Ben @bengregz was hit by a car on Friday and now on life support and a coma. He's the energy that we all love on the team. Sending all my prayers to you to fight this one .. you've got this Ben xxxx"
Fellow Welsh Dai Greene athlete wrote: "Sad to hear the news about @bengregz. Hoping he pulls through. One of the nicest and most positive people I've had the pleasure to be around at the track keep fighting Ben"
Curtis Mathews, who is also a Welsh decathlete, added: "This is a tweet for @bengregz it is deserved, more than most people I know. He is a very special man, brings joy to everyone he is around and deserves no less than the best from life.
"I can't describe how much I hope this champ pulls through because he is honestly the best of us. Please send your thoughts to this amazing man, he represents the best of us and is strong enough to pull through #TeamBenGregz"
