The man, in his 30s, an employee of the royal grounds in the east of England, fell into the water while sitting on a ride-on lawnmower. He was pulled from the water by police officers who then tried to resuscitate him before paramedics arrived.
"The man was retrieved from the water and was taken to Addenbrooke's Hospital by air ambulance for treatment. He is currently described as being in a critical condition," Norfolk police said.
A Buckingham Palace spokesperson said she is being updated on the incident.
The lake, which is around 200 metres wide with an island in the middle, is the largest of four inter-connected lakes and ponds in the 20,000 acre estate where the Royal family gather every Christmas.
The Queen, a well known Corgi lover and patron of the league, chatted with members and petted their dogs for around half an hour. She has at least four dogs of this kind. Her first Corgi was called Susan. Some portraits of Queen Elizabeth II include a Corgi.