A worker on the royal estate in Sandringham in Norfolk is believed to have been underwater "for a number of minutes" after the grasscutter he was riding went into a lake.
The man, in his 30s, had apparently become trapped underwater by the grasscutter. He was taken to Addenbrooke’s Hospital by air ambulance where he remains in a critical condition.
Emergency services were called to the scene at Lower Lake, close to the B1440 at around 1.30pm on Monday.
Two police officers and a colleague from the estate retrieved the man from the water and began CPR before paramedics arrived.
Norfolk Police and the Health and Safety Executive are continuing to investigate the accident.