The Queen's horse Estimate will miss the Melbourne Cup in November and have her final race in France or England.
Last year's Gold Cup winner will run in the Prix du Cadran at Longchamp on 5 October or Ascot's Long Distance Cup 13 days later.
The five-year-old, trained by Sir Michael Stoute, will then be retired from racing for a breeding career, said the Queen's racing advisor John Warren.
Melbourne had been mooted after the horse won the Doncaster Cup on Friday.
|Estimate: Career in figures|
|Races: 12. Wins: 5. 2nd: 2. 3rd: 2. Prize money: £454,853|
"After much consideration and in the best interest of the filly, it has been decided that Estimate will not be heading to the Melbourne Cup," said Warren.
Estimate was runner-up to Leading Light in the Gold Cup at Royal Ascot in June as she narrowly failed to defend her title.
But a month later it was announced she was one of a cluster of horses to return a positive test for morphine, which was blamed on contaminated feed.
She is likely to be disqualified from this season's Gold Cup at a British Horseracing Authority hearing on 25 September.
No British-trained runner has ever won the Melbourne Cup - the so-called "race that stops a nation" in Australia - which will be run for the 154th time at Flemington Racecourse on 4 November.