Charlie Appleby paid tribute to Masar after confirming the retirement of last year's Investec Derby hero.
The New Approach colt became the first horse to carry the royal blue silks of Godolphin to victory in the the premier Classic at Epsom last season, but a leg injury kept him off the track for the remainder of the campaign.
Masar made his long-awaited return in the Hardwicke Stakes at Royal Ascot last month - and performed with credit to finish fifth after stumbling soon after the stalls opened.
However, he disappointed when a hot favourite for last week's Princess of Wales's Stakes at Newmarket - trailing home last of six runners - and connections have now decided to call time on his racing career.
Appleby said: "After deep discussions with His Highness Sheikh Mohammed, the decision has been made to retire Masar.
"He's a horse who has given us many great days, including a day at Epsom last June that none of us will ever forget.
"From a personal point of view, he was my first Classic winner - and while it will be sad to see him leave the yard, we have to do what is the right thing for the horse, and I'm pleased he's retiring form racing fit and well."