Estimate, a racehorse owned by the Queen and trained by Sir Michael Stoute, has tested positive for morphine, a prohibited substance, Buckingham Palace announced.
Initial indications are that the positive test resulted from the consumption of a contaminated feed product, the palace said.
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The victims were playing the popular game in Whittington Park in Holloway on Tuesday night, when three males suspects approached them.
A former Royal Marine who was recently engaged has been killed in a climbing accident in the French Alps.
Through the day we'll see plenty of sunny spells but there will be a risk of showers too as they filter down from the northwest.