“The truth is the Duchess of Cambridge gets very seasick and doesn't enjoy the rocking motion of carriages," a royal source told the Sunday Mirror.
The Queen has even reportedly suggested medication to Kate Middleton, however the recommended medication can cause drowsiness.
According to the source, Kate was concerned about her sickness for Trooping the Colour.
"Protocol meant she had no choice but to go by state carriage."
This isn't the first time the Royals have discussed the problem of motion sickness.
In 2018, the Queen described the mandatory carriage ride for her 1953 Coronation as "horrible."
The two-hour journey involved riding in a 200 year old, 24ft, four tonne carriage being pulled by eight horses.
Buckingham Palace was "unable to comment" on Kate's seasickness.
Back in April, the Duke of Cambridge had to miss Prince Louis first birthday to go and fulfil his royal obligations in Christchurch, New Zealand after the terror attacks at Al Noor Mosque.