The Queen would have found celebrating the final two days of her Diamond Jubilee without the Duke of Edinburgh at her side "quite testing", according to Princess Eugenie.
Eugenie, speaking to Sky News about the final two days of the Jubilee celebrations, said: "They are the most incredibly supportive couple to each other. Grandpa was unfortunately taken ill and for granny to come and do that alone was probably quite testing and I think he is her rock, really, and she is his."
The Princess described the Jubilee as an "unbelievable experience".
She said: "To see that many people out there, I'm so amazed by the amount of love and support and gratitude that everyone has for granny.
"I was walking through the garden here the other day and someone came up to me and said 'I'd die for your grandmother, I think she's the most incredible person in the entire world to do what she's done for 60 years', and it makes me so proud.
The Princess was speaking before taking part in a 64-mile charity bike ride around London on Saturday night.
Eugenie, who suffers from scoliosis, had corrective surgery on her spine as a child and will be raising money for the Royal National Orthopaedic Hospital and the MCC Foundation to support the building of a cricket stadium in Rwanda.
Speaking about the potential difficulties she may encounter on the ride, Eugenie said: "What I'm worried about is the pressure it will have on my neck and my back because obviously doing this with rods in your back is not ideal but I want to show that it is possible."
Eugenie said she hoped she could be an inspiration to young people with similar conditions and described once meeting a patient with a similar condition.