BRITISH eventer Ros Canter and Allstar B lead the way after the opening day of the Mitsubishi Motors Badminton Horse Trials after posting a dressage score of 23.9 penalties.

Oliver Townend is currently seventh as he bids to complete the Rolex Grand Slam and eventing's £255,000 clean sweep.

Yorkshireman Townend will become only the third rider to achieve the feat after Pippa Funnell and reigning Olympic champion Michael Jung if he wins at Badminton.

Townend and Cooley SRS posted a dressage score of 25.9 penalties to go seventh overnight. He rides his current Burghley champion Ballaghmor Class on Friday and is among the main title contenders.

Behind early leader Canter, Germany's double Olympic champion Jung is second on La Biosthetique Sam FBW on 24.7 and Japanese challenger Yoshiaki Oiwa third with The Duke of Cavan.

America's Lauren Kieffer and Veronica is fourth, Funnell and Billy Beware in fifth, and Marlborough-based New Zealander Tim Price holds sixth spot on Ringwood Sky Boy with a score of 25.8.

Townend was among the early starters on a busy opening day at Badminton.

He arrived home from America on Monday after winning the Land Rover Kentucky event on Cooley Master Class, which set him up for a tilt at the Grand Slam.

Successive Burghley, Kentucky and Badminton victories constitute an eventing clean sweep.

Badminton also carries a £100,000 top prize, and Townend has already collected £105,000 from the combined efforts of Cooley Master Class and seventh-placed MHS King Joules in Kentucky, raising potential of a £460,000 windfall.

And assessing Cooley SRS' performance, he said: "I am over the moon with him, really happy. It is his first time at this level, and I was thrilled with his performance.

"The cross-country going (on Saturday) is going to be heavy and very testing, at best."

Overnight leader Canter, reflecting on Allstar B's display, added: "He is a very consistent performer. Every year, he gets stronger, more mature and keeps improving.

"I was so busy trying to concentrate today - I had no idea of the score. I am really proud of what we achieved."

And Jung said: "There are some great questions on the cross-country course, and it looks fantastic."

Further down the leaderboard, defending Badminton champion Andrew Nicholson, also from Marlborough, lies 18th with Nereo, but Saturday's cross-country phase is set to be pivotal in a likely congested race for Badminton's first place.

A number of major challengers will enter the fray on Friday, including Britain's Izzy Taylor, William Fox-Pitt and Piggy French, plus Ireland's Jonty Evans with his Rio Olympics horse Cooley Rorkes Drift.