Exciting colt Proisir is a long odds-on favourite to maintain his unbeaten run in Saturday's Group One Spring Champion Stakes at Randwick.

But David Payne, trainer of rival runners Honorius and Laser Flash, isn't conceding to the Gai Waterhouse-prepared colt just yet.

"He's not bred to stay so it will be interesting," Payne said.

"He is the best horse in the race but it's 2000 metres now."

Proisir is by globetrotting sprinter Choisir who didn't win beyond 1200m, although he was twice placed at Group One level over 1600m during his two and three-year-old seasons.

The colt's dam Prophet Jewel did not win beyond 1300 metres.

Proisir is unbeaten in three starts, his most recent victory coming in the Spring Stakes (1600m) at Newcastle which he won by almost six lengths.

TAB fixed odds posted him at $1.50 after the final field of seven was declared on Wednesday, ahead of It's A Dundeel at $4.60.

Honorius is third pick at $9 and Payne says he will strip fitter for his two runs from a spell which have yielded placings in the Ming Dynasty Quality (1400m) and Gloaming Stakes (1800m).

"His last run was very good, there was still improvement in him," Payne said.

"He will be better for it and the distance won't worry him."

Laser Flash will back-up from his third in last Saturday's Dulcify Quality (1900m), his first run in a month.

"He's a nice horse too. He's come on from his last run because he went into that with a month between races.

"The quick back-up won't be a problem for him."

Proisir will race himself into Cox Plate calculations if he can live up to his short quote and win well at Randwick.

He is currently a $15 chance for the weight-for-age championship and jockey Nash Rawiller says he has been ultra-impressive to date.

"He's a very exciting colt. He couldn't have been any more impressive than what he has been in his three starts," Rawiller said."