The actor, from Northern Ireland, was on holiday with a group including journalist and former showbiz editor Gordon Smart, who recounted the story during an appearance on The Good, The Bad And The Unexpected podcast.

He said that both he and Dornan fell ill while on the golf course after drinking alcohol including wine and espresso martinis the previous day.

Smart said he was the first to notice symptoms, including tingling in his left arm and hand, so he went into a medical centre and left his three friends behind, secretly fearing he might be having a heart attack.

He was told his heart rate was 210 beats per minute, he said - a normal resting heart rate should be between 60 to 100 beats per minute, according to the British Heart Foundation - and was put in an Uber to hospital.

"Got an Uber, collapsed and woke up in a hospital bed, attached to a machine, with a doctor saying, 'what on earth have you been doing, young man?'"

'That's not a good sign'

The doctor asked Smart to give details of the drinks he had had the day before, he said - at which point he apparently saw Dornan being rolled past on a hospital bed, too.

"I thought, that's not a good sign, that he's in the same state as me," Smart said. "At which point they left me in the room to calm down for a second."

He said he phoned his father, a doctor, who told him he might have caffeine poisoning after drinking six espresso martinis on top of the other alcohol they had consumed.

"I had to sort of ride it out," Smart said. "It was a pretty scary time."

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After they were both discharged, Dornan said he had begun feeling numbness in his limbs not long after Smart left the group.

About a week later, Smart said he received a phone call from the doctor who had looked after them.

"He said, look, espresso martinis aside, at any stage did you come in contact with caterpillars on the golf course?" Smart said, adding that the doctor sent him a link to a Portuguese newspaper article about processionary caterpillars.

"It turns out that there are caterpillars on golf courses in the south of Portugal that have been killing people's dogs and given men in their 40s heart attacks."

Caterpillars contain a toxic protein

The pair had apparently come into contact with a hairy processionary caterpillar, said Smart.

"I'd been very lucky to come out of that one alive. So there's my story. The good news is it wasn't a caffeine overdose, it wasn't a hangover. It was a poisonous, toxic caterpillar."

Dornan also survived the ordeal - and apparently even agreed to a selfie with staff at the hospital, Smart said.

"Don't ask Hollywood stars for a selfie when you think they've just had a heart attack," Smart joked at the end of the tale. "That was brutal, wasn't it? He did it as well. Good guy, good guy. He did the picture."

Processionary caterpillars are covered in hairs and move in groups, hence their name, following chemical and tactile signals on trees or the ground to find and consume leaves, the Natural History Museum says.

"This gives them safety in numbers, and their defence is boosted further by their covering of fine hairs," the museum says on its website. "Known as setae, they contain a toxic protein known as thaumetopoein which can cause rashes, irritation and breathing difficulties in humans and animals."

Dornan, best known for the Fifty Shades Of Grey films, as well as Belfast and A Private War, and TV series including The Fall and The Tourist, is yet to comment about the incident.