Leung waived his right for further independent testing of the 'B' sample and was immediately stood down from riding, pending a full inquiry into the test result on a date yet to be announced.

Leung's ketamine positive becomes the third to the drug since May 2009, when jockey Marco Chui Kwan-lai tested positive and was subsequently banned from riding for six months.

Chui returned to riding last November but again tested positive for the presence of ketamine in January this year and was suspended for a year at a subsequent inquiry.

Ketamine is widely used as an anaesthetic in both veterinary and human medicine but has become better known in recent years as a recreational drug.

Meanwhile, Ho opened his account with his first ever ride at the notoriously difficult track - winning the first event on Karbridge for trainer Caspar Fownes - before making it a double with a win on stablemate Achilles Of Troy in the fifth.

A third placing in the last also secured the Jockey Challenge for Ho - an impressive feat for a youngster riding his first races at the track.

Fownes piled on a treble of his own to clearly lead the trainers' table with five victories from 18 races this term after bagging the feature sprint with Perfect Gear care of a well-judged ride from Brett Prebble.

'Perfect Gear is a very good horse, he doesn't have to be ridden forward as he was tonight, he can settle and chase as well,' Fownes said.

'He's versatile in his style, but he can also win at Sha Tin, he's just a tough, honest racehorse.

'Tonight we were able to use his fitness to ride him a little more aggressively, and his fitness proved the difference in the end.'

Perfect Gear reeled in the speedy Super Boy to bag the Beech Handicap (1,200m) by a half length, with another half length to spare over the fast finishing Good Strike.

The Peter Ho Leung-trained All The Winners had a change of fortunes dropping to Class Five, with apprentice Ben So Tik-hung taking the until-now luckless gelding to the lead and driving him to a half-length maiden win.

'I think the drop in ratings and the better trip helped him a lot,' Leung said. 'He is not a star, but he has enough ability to win again this year.'

Champion jockey Douglas Whyte claimed his first win of the new season, guiding Happy Cha Cha to victory for trainer Me Tsui Yu-sak.

'It's good to get off the mark, Class Fives count as much as any other race, and it was no surprise that we got the good run from the ideal barrier,' Whyte said. 'His runs in class four last season were pretty good, and in my book he was a winner waiting to happen.'

A late thunderstorm, when 40mm of rain fell in half an hour, delayed the running of the final event - which was taken out by Golden Bauhinia on a downgraded soft track - and also hosed out the beer garden revellers who packed the course earlier in the night to cheer in the new season.

Road to ruin

Positive tests for ketamine among jockeys - including Kevin Leung (pictured) - in the past 16 months, 3