Britain's royal family has been thrust into a horseracing scandal - Queen Elizabeth II's filly Estimate has tested positive for morphine.

The Queen's racing adviser, John Warren, has confirmed her five year old is one of five horses that recently failed a British Horseracing Authority drug test.

A Buckingham Palace spokesman says early indications suggest that Estimate consumed the substance as a result of contaminated feed.

Warren states, "Her Majesty has been informed of the situation."

Estimate finished second in this year's (14) Gold Cup.