Sizing John will not be defending his Gold Cup title after being ruled out of the Cheltenham Festival.

Trainer Jessica Harrington has revealed that the eight-year-old, who was primed for a chance at becoming the first horse to successfully defend the Gold Cup since Best Mate in 2003, has a “hairline non-displaced fracture of his pelvis” which has forced him out.

Harrington now goes into the race one runner instead of two with Our Duke her sole runner.

Harrington exclusively told Unibet: “It is with a heavy heart I have to report that Sizing John will miss the Cheltenham Gold Cup. He cantered as normal this morning at Commonstown and during routine checks this afternoon was found to be sore behind.

“I have had our vet Mark McRedmond come and check him over. He has found a hairline non-displaced fracture of his pelvis. He will have box rest for two weeks and then be rescanned with a view of starting walking exercise . This is a massive blow for the Potts family and for all our team here at Commonstown.”

Bookmakers reacted by cutting favourite Might Bite even further and he is now 5/2 and William Hill admit that they are not too happy to see Sizing John sidelined.

“Might Bite is one of our worst results and we are gutted by Sizing John’s absence, he was perhaps our ace in the hole,” said William Hill spokesman Rupert Adams.