David Chapman-Jones is mourning the loss of Be My Royal, who died after a fall at Denge Wood near Waltham on Sunday.
The 14-year old bay gelding, who lived in retirement at his stables in Petham, had to be put down after suffering a broken shoulder.
An emotional Mr Chapman-Jones said: "It was the worst day my life. The horse was very special to me as it was the first one I treated and got him back racing again."
The horse fell into a ditch after colliding with another of Chapman-Jones’ charges while out exercising.
Be My Royal was a shock 33-1 winner of the Hennessy Gold Cup at Newbury in November 2002 but was disqualified after testing positive for morphine, found in poppy seeds in his feed. Then-trainer Willie Mullins took the case to the High Court but lost.
Feed company Red Mills then admitted responsibility, paid all the court costs and repaid the prizemoney.
Chapman-Jones added: "He came to me after rupturing a tendon in the race. We got him back racing a year later, and I owned him for the last two years when he ran in the Welsh Grand National at Chepstow and the Vodafone Gold Cup at Newbury."