WA stewards have launched a full inquiry after an accidental ring-in went undetected until after correct weight was posted for a Northam race on Thursday.
Ascot trainer Paul Jordan travelled with two runners: brown gelding El Alamein for a 1000m Maiden Plate, and bay gelding Urquharts Bluff for the Class 1 Handicap over 1400m.
In a mix-up, Urquharts Bluff was mistakenly saddled and took part in the maiden instead of El Alamein, winning the event before officials declared correct weight and only later uncovered the error.
El Alamein drifted in betting from $4.40 to $7.50.
The horse that originally finished second, $3.10 favourite Xentaro, will now be promoted to first.
As a consequence of the mistake, Urquharts Bluff was withdrawn from race six.