Mark Purdon and Natalie Rasmussen could have nine horses scratched if they do not return clear tests.

Betting on six races on Harness Jewels day at Cambridge on Saturday has been suspended by the TAB pending drug tests on nine leading contenders trained by Mark Purdon and Natalie Rasmussen.

The Jewels is one of harness racing's showcase events.

The leading trainers advised stewards on Wednesday afternoon the horses may have been exposed to drug contamination during a float trip from Auckland to Cambridge.

Stewards have ordered the nine horses be swabbed, but the results will not be known until Friday afternoon, the eve of harness racing's biggest event.

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It is believed the driver of the truck transporting the horses became concerned about activities of two workers travelling with the horses and reported their possible drug smoking.

Rasmussen contacted head harness steward Nick Ydgren on Wednesday.

"Natalie has phoned me concerned that some of their horses may have come in to contact with a contaminant which is prohibited under the rules," Ydgren told The Informant.

"We have decided our best course of action, on what she's advised us is to swab all nine horses for that substance."

Purdon and Rasmussen have addressed the issue on their All-Stars Racing Stables website.

The statement confirmed there was a remote possibility that contamination on the float trip could lead to irregular results in testing procedures during the Harness Jewels, so stewards were notified.

"We have advised the appropriate bodies of the possibility and all our horses will be urine tested on Thursday. We are not expecting any adverse outcome from those but we just want to be sure and keep everything out in the open," Rasmussen said.

"The incident goes back several days but we want to be absolutely sure there is no possibility of our horses being compromised as a result. We are not expecting withdrawals from any of the races. We are just taking every precaution," she said.

The horses affected include some of the hottest favourites on Jewels day, including Princess Tiffany, Winterfell, Sicario, Kayla Marie, Funatthebeach, Stress Factor, Cheerful, War Dan Delight and Bubbled Up.

The Informant has reported that Dibaba, from Terry and Glenys Chmiel's stable, will also be tested because she travelled with the nine Purdon-Rasmussen trained horses.

If the runners return positive swabs when the lab results are returned on Friday, the nine horses will be scratched from their Group I races at Cambridge.

Ydgren told The Informant closing the betting markets on five races was a tough decision but the only one that could be made.

"It's not a decision we took lightly; we discussed it at length and felt it was the best course of action."

The Purdon-Rasmussen stable has dominated harness racing in New Zealand for the last five seasons, winning nearly $20 million in stakes.