The Queen's horse Estimate claimed her first win since last year's Gold Cup with victory in the Doncaster Cup.
The 11-8 shot, trained by Sir Michael Stoute, was one of five horses to test positive for morphine in July.
John Warren, the Queen's racing advisor, said: "Estimate is such a game mare. The Queen would be so proud, she would have got a real buzz out of it."
The Melbourne Cup at Flemington in November is one of three possible autumn targets for Estimate.
Estimate, with Ryan Moore on board, crossed the line ahead of second-placed Whiplash Willie, with Kalann third.