The 81-year-old, who has not been named, suffered head injuries when a people-carrier with six other members of his family, from Kent, including his ten-year-old twin grandsons, crashed on Monday.
He was flown to Glasgow’s Southern General Hospital, after being seen at Western Isles Hospital in Stornoway, where his condition was described as critical but stable.
His 80-year-old wife was taken to the same hospital by air ambulance. She suffered spinal and facial injuries.
The silver people-carrier overturned in water at Loch Bearastadh Mor, just off the A859 Tarbert to Borve road, a mile north of Luskentyre on Harris.
The remaining five occupants – a 42-year-old driver, his 43-year-old wife, another adult aged 31 and the twins – were all treated at the scene for minor injuries.
Firefighters freed four trapped victims. The others had scrambled to safety.