Racing Demon won the Peterborough Chase for Knight

The nine-year-old, owned by Lady Lloyd-Webber, was rated a 33-1 chance for the blue riband in March.

Trainer Charlie Egerton said: "Darkness picked up a tendon injury in October and he won't be running this season. Hopefully he will be back next year."

King George VI Chase second and third Exotic Dancer and Racing Demon are also likely to swerve the Gold Cup.

The pair are being targeted at alternative Cheltenham Festival contests.

Entries for the Gold Cup closed on Wednesday, but will not be revealed until next week.

Darkness may not be among the entries, the Racing Post reported, along with others already ruled out such as Kicking King, Star de Mohaison and Celestial Gold.

Egerton's horse was third in the Royal & SunAlliance Chase at the 2006 Festival during a highly promising novice chasing campaign.

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Henrietta Knight's Racing Demon is being aimed at the Ryanair Chase at the Festival.

The horse had been as low as 16-1 for the Gold Cup after coming third to Kauto Star in the King George.

But Knight said: "I doubt Racing Demon will run in the Gold Cup this year. We will be looking at the Ryanair Chase."

King George runner-up Exotic Dancer will be entered in the Gold Cup, but could also run in the other race.

The seven-year-old, trained by Jonjo O'Neill, won both the Paddy Power Gold Cup and Boylesports.com Gold Cup at Cheltenham over shorter distances earlier in the season.

Barry Simpson, racing manager to Exotic Dancer's owner Sir Robert Ogden, said: "The Ryanair Chase is the obvious race for Exotic Dancer, but a lot depends on what else is in the race.

"Another option would be the Racing Post Plate, as that's a handicap worth �100,000, which is only �70,000 less than the Ryanair."

The Ryanair Chase is run over two miles, five furlongs while the Gold Cup is 3m 2f.

In previous years, the shorter race had been mooted as a potential target for both Kicking King and War of Attrition, who actually went on to land the bigger prize instead.