Trainer Nicky Henderson has said that Sprinter Sacre is "not a certain participant" in the BetVictor Tingle Creek Chase at Sandown on Saturday.
The brilliant seven-year-old is the hot favourite to stretch his unbeaten record over fences to 11 races, but has returned "unsatisfactory figures" from a tracheal wash which was taken on Monday.
Sprinter Sacre will be tested on Thursday morning before final declarations are made for the Grade One race to assess whether he can be given the go-ahead to run.
Henderson said: "We took what you can only consider to be a routine tracheal wash this morning. Although it appeared to be visibly satisfactory, the cytology from the lab has returned some unsatisfactory figures, which obviously gives us cause for concern. Although the horse is in fantastic form and his work on Saturday was, as ever, impeccable, I cannot ignore what these figures indicate.
"Although marginal, we have to issue a warning tonight, rather than any later, that he's not a certain participant on Saturday. I've spoken to [owner] Caroline Mould, who, like all of us at Seven Barrows, would not wish to take any chances. Therefore, we intend to repeat the procedure on Thursday morning ahead of declarations at 10am."
Last season's runaway Queen Mother Champion Chase winner has not been seen since his awesome victory at the Punchestown Festival in April. Henderson is adamant he will do nothing to place in jeopardy the long-term prospects of Sprinter Sacre, who will work again on Tuesday in the hope he can make the Tingle Creek line-up.
The trainer added: "Whatever happens, the long-term future for Sprinter Sacre is paramount. This is hopefully only a temporary blip, but it may sadly remove his first objective of the season.
"If it does, then I suspect the Victor Chandler Chase [at Ascot on January 18] will have to become the first port of call. In the meantime, he will continue his preparation as normal and he will work accordingly on Tuesday."
Sprinter Sacre is one of 11 entries for the Tingle Creek. Henderson has also entered Captain Conan, a winner of three Grade One novice chases last season, but he also holds an entry over three miles at Sandown on Friday.
Sire De Grugy is expected to take his chance while the Mick Channon-trained Somersby bounced back to form when winning Exeter's Haldon Gold Cup recently and has had this race as his aim since then.
Somerset trainer Paul Nicholls has the novice Hinterland and Kauto Stone entered. His Excellency, Oiseau De Nuit, Tataniano, Viva Colonia and the mercurial Mad Moose complete the list.
David Pipe's Junior has been left at the head of the weights for Saturday's Betfred Becher Handicap Chase at Aintree following the defection of Prince De Beauchene.
Junior has not run since Royal Ascot, when well beaten in the Queen Alexandra Stakes, and his sole experience of the unique Grand National fences did not last long when he fell at the second in the 2012 Grand National.
The ten-year-old will shoulder 11st 12lb if he lines up. Walkon, second to Hennessy winner Triolo D'Alene in the Topham, Big Fella Thanks and Roberto Goldback are next in the weights on 11st 8lb.
Pipe could also saddle Master Overseer, Bostons Angel and Swing Bill, while Donald McCain is considering the Becher for Across The Bay. Willie Mullins has two possibles in On His Own and Vesper Bell.
Planet Of Sound, Wyck Hill, Soll and Midnight Appeal are other interesting contenders.