Tom Dascombe's speedster was lining up in Saturday's Heed Your Hunch At Betway Handicap, after his planned appearance in the Listed Cleves Stakes two weeks ago was scuppered by a last-minute setback.
Connections faced a race against time to qualify him to defend his crown in the sprint division on All-Weather Finals Day on Good Friday, with options running out.
They chose to run in a handicap rather than a fast-track qualifier in France - but after Kachy broke in his usual electric style, he took a false step as the race began in earnest and was swiftly pulled up by Richard Kingscote.
It was later announced he had been put down.
Lowe tweeted: "RIP big fella. You were our world and will be missed forever. #KingKachy."
Kachy won nine of his 28 races, banking more than £470,000 in prize money.
As well as winning four times at Lingfield, including on 2019 Finals Day, he claimed the Group Three Molecombe at Goodwood as a juvenile and was twice placed in Group One company at Royal Ascot - finishing second in the 2016 Commonwealth Cup and third in last year's Diamond Jubilee Stakes.
Lingfield also paid tribute, the track tweeting: "We would like to offer our huge condolences to the connections of Kachy, who suffered a fatal injury in race three this afternoon.
"A true champion, he will live long in the memory of everyone at Lingfield Park."
The race was won by James Fanshawe's Harry's Bar (5-1).