The leading rider was aboard Thunder Home (14-1) who was attempting to make all, and had secured a sizeable advantage inside the final furlong.
However, Zero Error, who was making his debut for the John and Thady Gosden team, flew home from the rear of the field under Jamie Spencer, and collared Ryan's mount close home.
A nose separated the pair following a photo-finish, and the winner traded at 280-1 in running on the exchanges, while the runner-up was matched at 1.01.
The BHA stewards' report read: "Rossa Ryan, the rider of THUNDER HOME (IRE), had appeared to stop riding on the run to the line before recommencing his ride in the final strides to finish second, beaten by a nose. After being interviewed and shown recordings of the incident, Ryan was suspended for 28 days for failing to take all reasonable and permissible measures to obtain the best possible placing on a horse that would have finished first."
The Group One-winning rider had hoped to challenge for the jockey's championship this year, and is currently in third with 58 winners, but will now miss a big chunk of the summer following the dramatic incident.
Ryan's ban will begin on August 6 (until September 2) which means the leading rider will miss the Sky Bet Ebor Festival at York, live on Racing TV. Ryan was also expected to partner Royal Ascot winner Orthodox in the Prix Morny at Deauville on August 23.
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