The 25-year-old reporter, who was on holiday, had left his friends to use the toilet at Crocodile Rock, near Arugam Bay, in the east of the country at around 3pm.
Stunned friends said he was “waving his hands in the air” as the huge reptile grabbed him in its jaws and dragged him into the water.
Mr McClean was said to have been on a surfing trip organised by his hotel and was washing his hands in the lagoon, known for its large crocodile population, when the tragedy happened.
The journalist, who grew up in Surrey, had spent time covering Brexit and the EU for the British broadsheet and had recently returned to London after living in Brussels.
Before joining the paper he had graduated from Oxford with a First class degree in French.
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He had also written a moving blog about his brother, Neil, after the 22-year-old was diagnosed with leukaemia.
A Briton at the scene said: “There’s a lagoon right next to the sea. He went next to the lagoon and was grabbed by a crocodile.
“There are lots in the lagoon. People last saw his arms in the air in the water and then he was grabbed under.
“I was there but didn’t see it happen. Horrible.”
Fawas Lafeer, who owns the Safa Surf School, said: “He was learning to surf and after that he wanted to go to the toilet.
“He went in the jungle, about 800 meters. It was when he was washing his hands that the crocodile took him.
“They can’t do anything because the river is deep and murky, it is not very clear.
“They have sent out the Navy, Army and the task force, but I doubt they will find the body.
“The crocodiles take the bodies along river and hide them in the mud, so I don’t think he will be found until the day after tomorrow.”
After the incident, police were joined by navy and army units in a search of the shore and surrounding area.
In April this year, a 13-year-old girl was dragged away by a crocodile while on a day out with her family at Sri Lanka’s Pulnewa Lake, in Galnewa.
In July 2016, a Sri Lankan man aged 60 was killed in a crocodile attack while he was fishing.
Two years earlier a woman aged 57 died in another croc attack.
The Foreign Office said: “We are assisting the family of a British man who has been reported missing.”