Kate Middleton & Prince William Open Cancer Unit
Britain's Prince William and his wife Catherine, Duchess of Cambridge meet patient Ellis Andrews at the new Oak Centre for Children and Young People at The Royal Marsden Hospital in Sutton, Southern England Reuters

When the Duke and Duchess of Cambridge visited the Royal Marsden cancer hospital almost 30 years after Princess Diana's visit, Kate Middleton was spotted wearing her famous engagement ring.

The Royal Couple following the footsteps of Princess Diana prepared to open the £18 million Oak Centre for Children and Young People.

But two hours later when Middleton left the hospital, observers noticed that her engagement ring was mysteriously missing.

Prince William proposed to Middleton using his mother’s 28-year-old dazzling oval blue 18-carat sapphire and diamond ring

A palace representative told PEOPLE that the Duchess, following advice of the hospital staff, removed the ring from her finger for and kept it in her clutch.

The plan was always to take it off when she went into the ward in the same way the Duke and palace staff rolled up sleeves and tucked ties into their shirts, the source told People.

Hospital staffs were taking care of Kate’s clutch and William's jacket while the royal couple toured the Oak Centre.