The father of a toddler who has been missing since 1976 says he thinks about his son all the time and remembers the day he disappeared “like it was yesterday”.

Detectives have issued a fresh appeal for information about Sandy Davidson who is thought to have left his grandmother’s garden in Ayrshire to look for a family pet when he was three-years-old.

The child was playing at the home in St Kilda Street, Irvine, with his younger sister Donna, then aged two, when the dog escaped from the garden. The boy is believed to have followed but did not return.

Despite a search there has been no sightings of him since April 23 1976, the day he went missing.

Hs father, Phil Davidson, 66, said: “It has been 40 years but feels like 40 minutes. I still think about him every day. I still see him every day in my head.”

He added: “It was as if a spaceship came down, took him up and took him away. I think about Sandy all the time.”

Mr Davidson said hunting for his son was “like looking for a needle in a haystack” and urged anyone with new information to get in touch with police.

“Not knowing where he is kills me,” Mr Davidson said. “We are all in limbo. He was my first born.”

The child’s family has worked with charity Missing People on a series of campaigns aimed at finding Sandy.

Police said the charity will be using digital billboards to publicise pictures of the boy as he looked when he disappeared and a computer-generated image of how he might look now.

Chief executive of Missing People, Jo Youle, said: “To spend any length of time with a loved one missing is heartbreaking for a family desperate for news.

“Sandy's family have had to endure an unimaginable 40 years since Sandy disappeared.

“Everyone at the charity joins Police Scotland and the rest of the public in the hope that this new appeal will finally end the limbo that Sandy's family has been living in since the day he disappeared.”

Detective Superintendent David Halliday, the senior investigating officer, said: “It's hard to imagine the distress and sadness Sandy's family have endured over the last 40 years, not knowing what has happened to their beloved son and brother, who was only a toddler when he went missing.

“Despite the passage of time, this missing person investigation remains open and I'd like to take this opportunity on the anniversary of Sandy's disappearance to ask people to cast their minds back to Friday April 23 1976. Did you live in Bourtreehill in Irvine, specifically around the area where Sandy was last seen in St Kilda Street?

“Friday April 23 1976 was a relatively warm day as Sandy played outside with his sister Donna. The community of Bourtreehill was tight-knit and mostly everyone knew their neighbour.

“Did you see Sandy when he left his grandmother's garden? He was quite a distinctive-looking child with light blonde hair and blue eyes.”

Anyone with information is asked to contact Police Scotland by calling 101.