A worker is in a critical condition in hospital after being pulled from a lake on the Queen's Sandringham estate.
Emergency services were called to the Norfolk retreat at about 1.30pm on Monday after reports that an estate worker, in his 30s, had fallen into the Lower Lake while operating a ride-on lawn mower.
The estate worker is believed to have been underwater for a number of minutes
It is understood the Queen was at Sandringham on a private visit less than an hour later.
A spokesman for Norfolk Police said: "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.
"Police and the Health and Safety Executive are carrying out a joint investigation into the incident."
Lower Lake is next to the Estate Office, also known as York Cottage.
The converted house was the residence of King George V.
In a further statement, Norfolk Police said the man became trapped underwater by the lawnmower.
A spokesman added: "Two police officers and a colleague from the estate retrieved the man from the water and began CPR before paramedics arrived.
"It is believed he was underwater for a number of minutes and inquiries to establish exactly how long are ongoing."
The joint police and HSE investigation is focusing on speaking to witnesses and the technical aspects of the grass cutter.