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Estimate, the racehorse which has tested positive for a banned substance, helped make sporting history for the Queen as she became the first reigning monarch to win Royal Ascot's Gold Cup.
The Queen was pictured beaming as Estimate crossed the line in first place during the dramatic race last year.
Prime Minister David Cameron congratulated the Queen in a tweet saying: ''Huge congratulations to Her Majesty for Estimate's victory at Royal Ascot - first time ever a reigning monarch's horse has won the Gold Cup.''
Her Majesty won £155,960 in prize money after Estimate's victory.
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."
Tonight should be mild and muggy with temperatures as high as 17C in some places.
Tomorrow should be largely fine although there is a risk of showers and possibly the odd rumble of thunder as well later on.
Watch the latest weather forecast with ITV weather presenter Alex Beresford.
The Europe Air Safety Agency (EASA) said it will issue a "strong recommendation" to avoid Tel Aviv airport until further notice.
Russia probably has "really valuable information" on the MH17 plane crash, which left 298 people dead, and should "release what they know", David Cameron said.
The Prime Minister, who was speaking during a visit to the Shetland Islands, also welcomed the "good news" of fresh sanctions being imposed on Russia by the European Union.
He said: "I would make the point that the people who probably have the really valuable information about what happened to this flight are the Russians and they should release what they know about what happened over the skies of eastern Ukraine."
The United States will reveal intelligence later today that supports claims that the missile that downed the Malaysian Airlines plane came from a separatist-controlled area in Ukraine, the state department said.
More than 600 Palestinians have now died during the ongoing violence in Gaza as diplomatic pressure for a ceasefire intensified.
Twenty-seven soldiers are among the 29 Israelis killed in the conflict.
During talks with UN Secretary-General Ban Ki-moon, Israeli prime minister Benjamin Netanyahu compared Hamas to "al-Qaida" insisting the group must be dealt with.
ITV News Middle East Correspondent Geraint Vincent reports: