Carlton House has suffered a minor setback ahead of Saturday's Investec Derby at Epsom but is still expected to make the race.
Sir Michael Stoute's colt, who is owned by The Queen, was installed as favourite for Saturday's Classic after his Dante win at York on May 12.
However he was taken for x-rays on Tuesday morning after filling in his near-fore joint was spotted at evening stables on Monday night.
Her Majesty's racing manager, John Warren, explained on Tuesday: "Carlton House galloped on Sunday and did a nice six furlong blow, his last important blow before the Derby and on Monday he did a routine canter.
"At evening stables on Monday night Sir Michael noted that his front joints were not a pair and this morning he decided to take a precautionary x-ray of his near-fore joint which was the one that was marginally filled. It revealed no abnormalities which was a huge relief.
"He decided to give the horse just a trotting exercise this morning and he was sound and he's having an easy day. It will be a watching brief from now on. A small potential strain is the probability of what it was, just tweaked it a little, and hopefully he'll resume regular exercise for the rest of the week which will only involve cantering rather than anything faster.
"I would say at this point, from what I understand the injury to be, he has a pretty good chance of being there. The next few days will obviously reveal a little more but the trainer is optimistic and hopeful that the horse is still on target."
The news has sent the ante-post market into something of a frenzy, with the horse drifting with all bookmakers.
The favourite is now 7/4 from 11/8 with Sky Bet.
Matt Doyle of Sky Bet said: 'Although connections are being as positive as they can be and are hopeful Carlton House will line up at Epsom, it is a major setback for the Queen's horse with the Derby just four days away.'