'We've had the race in mind for him for a very long time' - Aidan O'Brien on his St Leger contenders

Aidan O'Brien knows exactly what it takes to win a St Leger. After all, he has landed Britain's oldest Classic seven times.

The Ballydoyle maestro, who fields three runners this year, including favourite Illinois, is adamant the ideal candidate will combine "class, stamina and speed".

It's invariably a tough race, and O'Brien believes a three-year-old needs to stay the extended mile and three-quarters really well and have the class to get into it.

He is confident Illinois, the unbeaten Jan Brueghel and recent 20-length scorer Grosvenor Square all possess the right credentials.

Illinois proved his stamina by winning the Group 2 Queen's Vase over 1m6f at Royal Ascot before showing the necessary class when going down by only a neck to Irish Derby hero Los Angeles in York's Group 2 Great Voltigeur Stakes.

"We've always thought Illinois was a St Leger horse," O'Brien said. "He stays very well and we've had the race in mind for him for a very long time.

"We were delighted with him at York and we know he will stay this longer trip well. It's important he gets nice ground so hopefully he will."

Punters are instinctively drawn to an unbeaten horse and Jan Brueghel, a short price on bare achievement but with the potential for significant improvement at this longer trip built into his odds, has won all three starts.

The son of Galileo landed the Group 3 Gordon Stakes at Goodwood despite appearing not to handle the ups and downs of the Sussex track particularly well.

He should be far happier on Doncaster's flat, galloping circuit, and his trainer feels Jan Brueghel has improved significantly since.

"We think this longer trip will suit Jan Brueghel and we also think he has improved a good bit since Goodwood," O'Brien revealed. "I think the long, flat straight will really suit him and he seems in good form."

Grosvenor Square: St Leger is on the agenda

Grosvenor Square won the Irish St Leger Trial by 20 lengths on his most startCredit: Patrick McCann (racingpost.com/photos)

Grosvenor Square might be the biggest price of O'Brien's three runners, but he certainly looked to have a nice blend of class, stamina and speed when making all over 1m6f at the Curragh last time.

O'Brien said: "Grosvenor Square is an unusual horse in that he seems to have an awful lot of stamina but he is a very high cruiser at the same time.

"He'll go out there and make the Leger a very tough race. We've been very happy with him since he won at the Curragh."

What they say

Owen Burrows, trainer of Deira Mile
I've been very pleased with him since Windsor. If he reproduces his fourth in the Derby, he should go to Doncaster with a good each-way shout. I'm hoping the extra distance will suit and think he'll like the track. Obviously he wouldn't be good enough judged on that Windsor run, but it was his prep race for the St Leger and it turned into a messy four-runner race. He was only a length and a half behind Los Angeles at Epsom and if Los Angeles was in the St Leger I'm sure he'd be a shorter price than Illinois.

David Menuisier, trainer of Sunway
He came out of the King George very well. He passes the line strongly and Oisin [Murphy] and James [Doyle] are both confident he will stay the trip and could even improve for it. The horse is in good shape.

Ralph Beckett, trainer of You Got To Me
I'm glad we're running. She's in good shape and I think the race will suit her very well. If she's in the shape I think she's in, she should be very competitive.

St Leger day previews:

1.50: 'I think he'll win' - can Vintage runner-up Wolf Of Badenoch go one better in Group 2 Champagne Stakes?

2.25: 'He goes there with a live chance' - trainer quotes and analysis for the Portland Handicap

3.00: Kinross helped by rain - but younger competition snapping at his heels in Park Stakes

Is there anyone who can fill Ryan Moore's giant boots? Graeme Rodway crunches the numbers on Ballydoyle's substitutes

How much faith do you have in the favourite? Aidan O'Brien sets a St Leger puzzle for punters and rivals