Daryl Jacob was handed a 10-day ban by the Wincanton stewards for failing to ride out to the line on Ulis De Vassy in the Racing Welfare Fox-Pitt Eventing Handicap Hurdle.
Jacob is set to miss the lucrative Christmas period, where he could have expected to be riding plenty of top-class horses for Paul Nicholls, who employs him as second jockey to Ruby Walsh.
The dates of his suspension mean he will sit out 20-22, 26-31 December and 1 January.
Jacob was in line for the mount on Kauto Stone in the King George VI Chase at Kempton on St Stephen’s Day after partnering him to victory at Down Royal last month. He will also miss Chepstow's Welsh National meeting.
Jacob was in front over the last flight on the Nick Williams-trained Ulis De Vassy but was headed inside the last 100 yards by Quaddick Lake.
It appeared that Jacob mistook a path, several strides before the line, as the winning post, before realising his error and riding out for a couple of strides more.
The stewards punished him under Rule (B)59.4 as they believed he still could have won. The official winning margin was a neck.
Paul Struthers, chief executive of the Professional Jockeys Association, will consult with Jacob over the weekend and seek a second opinion before deciding whether or not to appeal.
"I've seen the race on TV but I am collecting a DVD this evening when we will have a better look at it," said Struthers.
"You can slow it down and watch it in more detail with a DVD, so it is worth doing.
"I appreciate there are two elements to the rule 'would have won or could have won' but I think he was beaten anyway.
"I don't think what Daryl did made any difference to the result, but we will get a second opinion anyway.
"We have until the close of play on Monday to decide whether to appeal. We'll discuss it over the weekend."