The eight-year-old carried all before him last season, but this campaign has not gone to plan after an impressive return in the John Durkan Memorial Chase at Punchestown in December.
The horse was owned by Yorkshire businessman Alan Potts and his wife Ann, of Barnsley, who have both died since Sizing John’s memorable triumph at Prestbury Park 12 months ago.
He was sent off the hot favourite after that to win the Leopardstown Christmas Chase but was well-beaten in seventh place, and although Harrington was hopeful he was returning to his best for the Festival, a “hairline non-displaced fracture of his pelvis” has dashed plans.
Harrington said: “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.”
Harrington is still set to be represented in the Gold Cup through Our Duke.
Ruby Walsh made a successful return from his lengthy spell on the sidelines when steering the Willie Mullins-trained Lareena to a clear-cut victory at Thurles.
Walsh had been out of action since suffering a broken leg in a fall from Let’s Dance at Punchestown on November 18, but has always maintained he would be back in time for next week’s Cheltenham Festival where he has been leading rider on 11 occasions.
Regarding his fitness, the self-deprecating jockey said: “I thought I was all right anyway – she won didn’t she!
“We’ll see what Willie decides to do (over the coming days before Cheltenham) but I don’t mind, and I’ll do whatever Willie wants me to do.
“When you’re back you’re back and he’s my boss now again!”
One of Walsh’s most high-profile rides is likely to be aboard Douvan who is set to clash with Nicky Henderson’s two mile superstar Altior in next Wednesday’s Queen Mother Champion Chase.
Douvan and Faugheen were reported in “great form” after working yesterday as the countdown to Cheltenham gathers pace.
The Mullins-trained stars are expected to feature in the Betway Queen Mother Champion Chase and Unibet Champion Hurdle respectively, although both have questions to answer.
Douvan has not run since suffering a shock defeat in last year’s Champion Chase, in which he picked up an injury, while Faugheen was last seen finishing second to Supasundae in the Irish Champion Hurdle at Leopardstown.
Mullins’ son and assistant Patrick said: “Douvan and Faugheen are both in great form and both worked great this morning and it’s all systems go.
“Min is in the Champion Chase and Un De Sceaux is in the Ryanair, so I don’t know about (race plans for) Douvan, but he’s in great form.”
Amateur rider Kane Yeoman’s first ride for the in-form Sue Smith stable was a winning one when Red Danaher, owned by the trainer and her husband Harvey, landed the opener at Carlisle.
Never afraid to give young riders a chance on their own horses if they’ve earned the opportunity to impress, it was also Yeoman’s first ever winner under National Hunt rules.
Beverley, Ripon and York racecourses have been named amongst the top 12 tracks in Britain.
It follows an annual assessment by the Racecourse Association to check customer care at the country’s racing venues.
Beverley chief executive Sally Iggulden said: “We continually aim to offer the highest level of customer care, and to be recognised as a leading player amongst the competitive racecourse industry is wonderful for the whole team.
“Not ones to rest on our laurels, we will build on this news and continue to aim to make Beverley Racecourse the very best day out for all of our visitors”.