The 15-times champion jockey has been booked by trainer Paul Nicholls after Fehily stood himself down following the recurrence of a wrist problem.
Fehily had been set to replace Ruby Walsh, who is recovering from a leg injury, but the wrist injury he suffered in a fall at Newbury last month has flared up again after he rode at Kempton on Tuesday.
Nicholls revealed on twitter.com last night that McCoy would partner the dual Cheltenham Gold Cup hero in his bid to win the Christmas showpiece for an unprecedented fifth time.
Walsh partnered the Clive Smith's ten-year-old to his four previous King George successes.
McCoy, who landed the big race on Best Mate in 2002, has now capped a memorable few days after being voted BBC Sports Personality of the Year on Sunday.
Expressing his sympathy for Fehily, who Kauto Star trainer Paul Nicholls said is set to be sidelined for six weeks because of the wrist problem, which requires surgery, McCoy said: "That is the worst part of being a jump jockey. Obviously Noel was going to ride him and before that Ruby Walsh but luckily for me I've been given the opportunity.
"You always want to ride the best. When Paul asked me my immediate thought was, 'Of course I want to ride him' but you've got to speak to Jonjo O'Neill and Trevor Hemmings.
"I rang Jonjo and I rang Trevor Hemmings myself. I wasn't being very loyal - I wanted to ride another horse but he was very understanding and luckily for me that was the case.
"He's a fantastic horse, he stays well and if he puts up anything close to his King George performances beforehand he'll win it."
Nicholls told BBC Radio Five Live: "I was disappointed for Noel, I didn't think he was particularly fit yesterday and I wasn't terribly surprised.
"I have got a lot of respect for the guy - not everybody would do that so you have got to take your hat off to him."
Asked how he had secured McCoy as Fehily's replacement, Nicholls added: "Quite simply I picked the phone up and asked if he was free, and if he didn't have anything else he would be top of the list and left it to him.
"I had to ring him and offer him the ride, he then had to chat to Trevor Hemmings and Jonjo O'Neill and Trevor let him off Albertas Run to ride Kauto so you have to do it right."
A devastated Fehily said: "It's the wrist injury I had earlier. It had been good for my comeback and everything was great, but I had a couple of rides yesterday and I've tweaked it again somehow. It's just flared up again and I haven't got enough time to get it right. I need to get it looked at again. It's not good."The weather, of course, continues to cast doubts on whether the race will actually take place. But there was a noticeable improvement in conditions at Kempton yesterday. Clerk of the course Barney Clifford describes the chances of racing at the weekend as "50-50" after a "good thaw" and says they will pull out all the stops to try to ensure the meeting goes ahead as scheduled. "One would have to be optimistic," said Clifford.