Frankel's bid for 14 victories from 14 races could be undone by the weather.
The world's top-rated racehorse goes in the Champion Stakes at Ascot on Saturday but the course has been hit by rain in the build up to the weekend.
Another five millimetres overnight and predictions of further showers mean the ground could go from soft to heavy.
With Frankel having not raced on ground worse than good to soft since his debut, Sir Henry Cecil, the horse's trainer, has doubts as to how the horse will perform, "I'm pretty confident he will be fine in soft ground but if it's heavy, we are in no man's land,
"He has never encountered it and with his action and turn of foot, I cannot be sure he would appreciate it."
Cecil's unbeaten four-year-old, will face last year's winner, Cirrus Des Aigles, and the talented colt Nathaniel in the £1.3m race.
The Champions Stake is expected to be Frankel's final race before he retires to stud.
The race is Europe's most valuable mile-and-a-quarter race and the highlight of British Champions Day - the richest day in British racing.