The Queen has been informed of her racehorse's failed drugs test, her majesty's racing advisor said.
Estimate, which won the Gold Cup at Royal Ascot in 2013, tested positive for morphine - a prohibited substance.
Sir Michael Stoute, the racehorse's trainer, is offering his "full co-operation" to the investigation, John Warren, from the Queen's bloodstock and racing advisor, said.
"Sir Michael is working closely with the feed company involved to discover how the product may have become contaminated prior to delivery to his stables,
"As the BHA investigates this matter, including potential links between the different cases, Sir Michael continues to offer his full co-operation.
"Her Majesty has been informed of the situation."
More top news
The term 'photobomb' has been named the Word of the Year 2014 by lexicographers at Collins English Dictionary.
A Swiss supermarket has apologised for its "unforgivable blunder" after selling coffee cream with pictures of Hitler and Mussolini on.
The Tonight programme looks at government efforts to improve our general wellbeing, and offers tips on how we can all boost our happiness.