Benjamin Mullany, 31, is in hospital on the island of Antigua with a bullet lodged in his brain after he and his bride, Catherine, also 31, were shot in a suspected robbery.
Dr Fidel Fernandez, a surgeon at Holberton Hospital, said: "The situation is that Mullany is critically ill – brain-dead."
"There is no activity. The brain is dead. He is on a ventilator which supports his organs and without that he would die.
"There is no chance of survival. It is very, very sad."
Mr Mullany's parents, Marilyn and Cynlais, arrived on Antigua on the day the newly-weds had been due to begin married life together at their home in Rhos, near Neath, in Wales.
The young couple were shot by one or more intruders at their cabin at the luxury Cocos Hotel at around 5am on Sunday morning. Mr Mullany, a physiotherapist was shot in the neck and the bullet passed into his brain, while his wife was shot in the head.
Police have arrested six suspects on the island, but have admitted they have no clear leads in the investigation.
Inspector Cornelius Charles said: "I wish Benjamin would come around and tell us because, barring that, this is a mystery"
He said nobody at the hotel had seen any suspicious characters either enter the grounds or leave them at the time of the murder. There was no evidence of any sexual attack on Mrs Mullany, he added.
A fellow guest at the hotel has told police she heard Dr Mullany cry for help just before she died.
In a statement to Antiguan police, Lorraine Martin-Bell said she and her husband, Steve, heard Dr Mullany scream three times and shout loudly: "Help me please!" before two shots were fired in their hotel cottage.
Mrs Martin-Bell, from Devon, said they had been staying in the cottage next door when she was woken at around 4.45am by what she thought was music. She then heard a female scream and briefly went outside, but dismissed it as "someone messing around" and went back to bed.
Her husband then woke up and they both heard a second scream. They both went to the door and looked out but saw nothing.
"We went back inside and secured our door, and that was when I heard an explosion that sounded like a gunshot," she said.
"Immediately after that I heard a female scream louder than before. Shortly after, I heard another explosion and a female voice shouted, 'Help me please!' in a loud tone of voice. And then everything went really quiet after that."
She said she saw nobody run from the room and nothing unusual about it, except that the balcony light was on. They only realised something was badly wrong when they heard loud talking outside the room at about 6am.
Colleagues of Dr Mullany, who worked in the A & E department at Morriston Hospital, Swansea, until February, said she had talked about her wedding for months before hand.
Michael McCabe, a consultant in the department, said: "Catherine was pleasant, smiling and someone you could get on with, and relax with, someone everyone had a rapport with, staff and patients alike."
"She was special; you could always depend on her.
"She gave 110 per cent all the time. If there was extra work to be taken on she would take it on."
Dr Dewi Evans, a paediatrician, who worked with Dr Mullany at Singleton Hospital, Swansea, said: "I have known the family for a long time and no-one could wish for a better daughter than Catherine, nor a better doctor.
"She could have succeeded in any discipline of medicine but she wanted to be a GP because that suited her plans for her future family life."
He added: "There is a cloud hanging over the hospital but Catherine would have wanted us to carry on putting the patients first. That is the sort of person she was."
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