Frankel will have to overcome his biggest test in the Champion Stakes at Ascot on Saturday if he is to retire unbeaten in 14 races, the racing manager for owner Prince Khalid Abdullah says.

Teddy Grimthorpe was speaking after the latest forfeit stage left just seven rivals to oppose the Henry Cecil-trained superstar who will be tackling 1-1/4 miles (2000m) for only the second time in his career.

However, the opposition will be tougher than he faced in the Juddmonte International at York in August with France's winner from last year Cirrus des Aigles topping the list, having returned to the track after a four month break and won the Group Two Prix Dollar this month by nine lengths.

Also in the mix are Eclipse Stakes winner Nathaniel, who held the four-year-old colt to his shortest winning distance of half a length length in his first start two years ago, and German Derby winner Pastorius as well as the Aga Khan-owned filly Ridasiyna, who won the Group One Prix Opera on October 7.

"Certainly this is his toughest test," Grimthorpe told At The Races.

"It's building up to be a really true test for him. I think it's exciting.

"I've always come into this with the attitude that we have to enjoy him. The horse has done everything that we could possibly have dreamt about and I hope he'll run a huge race on Saturday."

Frankel, who will as usual have his older brother Bullet Train setting the pace for him, will likely also have to race on his least favoured soft ground but Grimthorpe said this would not be too much of a problem.

"I would think that soft wouldn't be too much of a problem," he said.

"It was quite soft when he worked on the racecourse but (work rider) Shane (Fetherstonhaugh) got off him and said he handled it very nicely. Henry is very pleased with him. It's all systems go."