A source said the son, a Form Six pupil, was moderately intellectually challenged and autistic.

Their last known location, based on a mobile phone signal, was on Sai Sha Road.

The force was alerted at 12.31am on Sunday by the 61-year-old’s wife, who said the two had not returned home after they headed to Sai Kung for a hike at around 10am the previous day and she was worried they had run into trouble.

The source said the woman was unable to give more information on the pair’s hiking route, although she told officers the two normally returned home before 7pm when they went out on the hills.

Officers from the force’s Ma On Shan and Sai Kung division and Fire Services Department personnel started the search in the Tseng Tau Village and Shui Long Wo areas on Sunday morning.

Government Flying Service helicopters were also deployed to help in the hunt for the missing men.